key: cord-0760997-j26isz1n
authors: Walker, Lucy S. K.
title: The link between circulating follicular helper T cells and autoimmunity
date: 2022-03-11
journal: Nat Rev Immunol
DOI: 10.1038/s41577-022-00693-5
sha: 4da9908331e92bb3d5932568a12f9a6a67d7af46
doc_id: 760997
cord_uid: j26isz1n

Follicular helper T (T(FH)) cells provide help to B cells, supporting the formation of germinal centres that allow affinity maturation of antibody responses. Although usually located in secondary lymphoid organs, T cells bearing features of T(FH) cells can also be identified in human blood, and their frequency and phenotype are often altered in people with autoimmune diseases. In this Perspective article, I discuss the increase in circulating T(FH) cells seen in autoimmune settings and explore potential explanations for this phenomenon. I consider the multistep regulation of T(FH) cell differentiation by the CTLA4 and IL-2 pathways as well as by regulatory T cells and highlight that these same pathways are crucial for regulating autoimmune diseases. The propensity of infection to serve as a cue for T(FH) cell differentiation and a potential trigger for autoimmune disease development is also discussed. Overall, I postulate that alterations in pathways that regulate autoimmunity are coupled to alterations in T(FH) cell homeostasis, suggesting that this population may serve as a core sentinel of dysregulated immunity.

Coordinated interaction between different immune cells is crucial for the development of protective immunity. Nowhere is this more obvious than in the emergence of high affinity antibody responses, where carefully orchestrated contacts between dendritic cells (DCs), T cells and B cells culminate in a refined and long-lived antibody response. The critical go-between in this cellular trio is the follicular helper T (T FH ) cell that liaises first with the DC, before migrating to the B cell follicle for repeated interaction with B cells 1 . Such T FH cells bear a characteristic phenotype including expression of markers such as CXC-chemokine receptor 5 (CXCR5; which promotes homing to the B cell follicle), programmed cell death protein 1 (PD1), inducible T cell costimulator (ICOS) and the transcription factor BCL-6 (ref. 2 ) (Box 1). T FH cells are classically found in secondary lymphoid organs with a small population of similar cells present in the blood (referred to as circulating T FH (cT FH ) cells). Curiously, multiple studies have revealed that cT FH cells are present at increased frequencies in many autoimmune diseases, including systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), Sjogren syndrome (SS), autoimmune thyroid diseases, myasthenia pathology and robust development of T1D when crossed to a T cell receptor (TCR) transgenic mouse model. Impairing T FH cell differentiation, by rendering sanroque mice heterozygous for BCL-6 or deficient for SLAM-associated protein (SAP, an adaptor protein required for T FH cell-B cell interactions 5 ), ameliorated the autoimmune phenotype, leading to reduced autoantibody production and decreased renal pathology 6 . These findings sparked interest in whether T FH cell differentiation might be connected to lupus pathology in humans, leading to the demonstration that cells with a T FH cell phenotype were elevated in the blood of patients with SLE 7 . Reports documenting cT FH -like cells in numerous other autoimmune diseases rapidly followed (reviewed in ref. 8 ).

The spotlight was then turned on the true identity of these blood-borne T FH -like cells, specifically on their relationship to germinal centre (GC)-resident T FH cells. Important work from the Ueno group revealed that circulating CD4 + CXCR5 + T cells shared functional properties with T FH cells from secondary lymphoid organs and could provide help to B cells via IL-21 production 9 . This led to the idea that blood-borne CD4 + CXCR5 + T cells were a memory counterpart of lymphoid T FH cells, and a substantial body of evidence has since emerged to support this view [10] [11] [12] [13] [14] . Consistent with this notion, individuals with defects in GC formation due to deficiency in ICOS or CD40L, or a developmental block in B cell development (BTK deficiency), have fewer circulating CD4 + CXCR5 + cells, and they are absent altogether from the cord blood of newborns 15, 16 . Of note, many T FH cell markers are downregulated in the memory phase 12, [17] [18] [19] , perhaps explaining early controversies over the existence of a T FH cell memory pool. Experiments involving the adoptive transfer of mouse T FH cells into antigen-free hosts indicate that CXCR5 expression is least affected by the phenomenon of T FH cell marker loss 20 , suggesting it may be the most reliable marker for tracking cT FH cells. Painstaking experiments in which cT FH cells were purified from mouse blood revealed that these cells can home to secondary lymphoid tissues upon re-challenge and participate in GC reactions, providing direct evidence gravis, type 1 diabetes (T1D) and multiple sclerosis (MS) 3 . Consistent with elevations in cT FH cell numbers, autoantibodies are commonly associated with these conditions and their presence often precedes symptomatic disease.

In this Perspective, I discuss core mechanisms controlling T FH cell differentiation and highlight that these same pathways are linked to the regulation of autoimmune disease. I also discuss the potential for infection to serve as a link between T FH cells and autoimmunity. Overall, I postulate that regulation of T FH cells and regulation of autoimmunity are tightly coupled, perhaps explaining why increases in cT FH cell numbers are evident across multiple autoimmune diseases. 4 . The causative mutation in these animals mapped to the Roquin (Rc3h1) gene, the product of which repressed ICOS expression and negatively regulated T FH cell differentiation. These sanroque mice exhibited lupus-like of their functional capacity upon antigen re-encounter 18 . Thus, the consensus view is that T FH cells have a circulating memory compartment that retains expression of CXCR5 and, to a variable extent, other T FH cell markers and can be recalled to participate in the memory phase of humoral responses.

Somewhat unexpectedly, the majority of cT FH cells do not seem to derive from the GC environment itself. In fact, imaging studies have shown that, although GC T FH cells frequently move between GCs, their egress to the circulation is rare 21 . Instead, it appears that circulating CXCR5 + cells arise mainly from T FH cells that have not yet undergone sustained interaction with B cells. In line with this, the frequency of cT FH cells appears undiminished in mice and humans that lack SAP expression and consequently exhibit impaired T cell-B cell interactions 22 . The link between lymphoid tissue T FH cells and cT FH cells has been compellingly demonstrated both by mass cytometric comparison of blood and tonsillar tissue with dimensionality reduction approaches 23 and by vaccination studies showing that blood CD4 + CXCR5 + ICOS + PD1 + cells 24 or CD4 + CXCR5 + PD1 hi cells 25 are clonally related to lymph node GC T FH cells. Overall, this suggests a model in which T FH cells at the T cell-B cell border can give rise to both GC-resident T FH cells and cT FH cells that travel through efferent lymph to the blood ( fig. 1 ). Consistent with this, T cells with T FH cell features can be detected in thoracic duct lymph of mice and humans, and treatment with the drug FTY720, which prevents lymph node exit, dramatically decreases cT FH cells in both species 18, 26, 27 .

Common pathways regulate T FH cell differentiation and autoimmunity T FH cell differentiation and provision of help to B cells is a tightly controlled process. B cell tolerance relies heavily on restricting T cell help, in part because the developmental regulation of T cells is more stringent; nascent T cells are screened against self-antigens during their thymic development, with transcription factors, such as autoimmune regulator (AIRE), ensuring a broad representation of peripheral self-antigens. By contrast, peripheral self-antigens may be less available to B cells developing in the bone marrow, and it has been reported that around 20% of mature naive B cells exhibit low levels of self-reactivity 28 . B cell antigen specificity is also prone to diversification within GCs, superseding any developmental constraints on self-reactivity. Thus, denying T cell help to self-reactive B cells is necessary to prevent the initiation of potentially damaging autoimmune humoral responses. Ensuring that T cell-B cell collaboration is tightly regulated is also important for optimal protective immunity (Box 2).

In considering the mechanisms controlling T FH cell differentiation, three core and interconnected pathways emerge involving cytotoxic T lymphocyte antigen 4 (CTLA4), regulatory T (T reg ) cells and IL-2. Curiously, these same pathways are well recognized for their ability to regulate autoimmunity. Below, I explore evidence indicating that these pathways control T FH cell differentiation and highlight their connection to autoimmune disease susceptibility.

BCL-6 is an essential transcription factor for follicular helper T (T FH ) cell differentiation [143] [144] [145] , and its repression of BLIMP1 is necessary 144 but not sufficient 146 for the T FH cell fate. Other key transcription factors include MAF, which is upregulated by inducible T cell costimulator (ICOS) ligation 147 , and BATF, which controls expression of BCL-6 and MAF 148, 149 . ASCL2 promotes early T FH cell differentiation by upregulating the expression of genes such as CXCR5 (ref. 150 ), whereas repression of KLF2 expression is required to maintain the T FH cell phenotype 151 . STAT proteins play a major role in influencing T FH cell differentiation in response to cytokines: STAT5 inhibits T FH cell differentiation following IL-2 exposure and, conversely, STAT3, STAT1 and STAT4 promote T FH cell differentiation in response to cytokines such as IL-6, IL-21 and IL-12 (refs 81,152,153 ). 16 . Blocking lymph node exit, by treatment with FTY720, dramatically decreases cT FH cells in mice and humans 18, 26, 27 . Clonotype sharing reveals a developmental relationship between T FH cells found in the GC and the blood 24, 25 and those found in the lymph and the blood 26 , suggesting that T cell-B cell interactions at the T cell-B cell border give rise to both GC T FH cells and cT FH cells. It is possible that some GC T FH cells enter the circulation from GCs but this is likely to be rare 21 . Follicular regulatory T (T FR ) cells also have circulating counterparts (cT FR ); however, these have an immature phenotype and differentiate from regulatory T cells without the requirement for contact with B cells 164 .

www.nature.com/nri P e r s P e c t i v e s 0123456789();: There is also evidence that the CTLA4 pathway regulates T FH cells in humans. Individuals deficient in LRBA, that show defective CTLA4 trafficking and function, exhibit increases in circulating T cells expressing T FH cell markers (CXCR5 and PD1) 34 . In addition, patients with cancer receiving anti-CTLA4 antibody immunotherapy show an increase in circulating T cells with T FH cell markers (E. Ntavli, N. M. Edner and L.S.K. Walker, unpublished observations). Conversely, cT FH cells are decreased following treatment with soluble CTLA4 molecules, such as the CTLA4-immunoglobulin fusion protein abatacept, in individuals with SS 35 , RA 36 , MS 37 or T1D 38 . Thus, the CTLA4 pathway negatively regulates T FH cell homeostasis in mice and humans, likely by restricting CD28 engagement.

CTLA4 and autoimmunity. The association between CTLA4 and autoimmunity is well documented. Genetic variation at the CTLA4 locus is linked to numerous autoimmune diseases, including T1D, RA, SLE, myasthenia gravis, autoimmune thyroid diseases, coeliac disease, alopecia areata and vitiligo (see GWAS Catalogue). Mice genetically deficient for Ctla4 develop lethal lymphoproliferation and multiorgan immune cell infiltration 39, 40 , and heterozygous CTLA4 mutations in humans are associated with an immune dysregulation syndrome with multiple autoimmune manifestations 41, 42 . Targeting the CTLA4 pathway by immunotherapy in patients with cancer can also elicit autoimmune side effects. CTLA4 function may be altered indirectly by mutations in genes encoding CTLA4 pathway regulators. For example, mutations in LRBA lead to reduced CTLA4 expression and autoimmune outcomes 43 .

T reg cells express the transcription factor FOXP3 and play a crucial role in the maintenance of immune homeostasis. Scurfy mice, which lack functional T reg cells owing to a frameshift mutation disrupting Foxp3, exhibit a marked expansion of BCL-6 + CXCR5 + T FH cells in secondary lymphoid tissues 44 . Consistent with this, in mice expressing diphtheria toxin receptor under the control of the Foxp3 promoter, short-term depletion of T reg cells enhances the generation of antigen-specific T FH cells in response to immunization 45, 46 . Similar to mice, patients with IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked) that have mutations in FOXP3 also exhibit an increased frequency of CXCR5 + PD1 + cT FH cells 47 . Thus, FOXP3 + T reg cells appear to control T FH cell numbers in both mice and humans.

T reg cells constitutively express CTLA4. Interestingly, the enhanced T FH cell differentiation associated with CTLA4 deficiency 29 can be recapitulated by loss of CTLA4 expression in T reg cells alone 45, 48 . To avoid widespread immune dysregulation, Sage et al. 48 used mice in which tamoxifeninducible Foxp3-Cre was used to excise the floxed Ctla4 gene in T reg cells immediately prior to immunization, whereas Wing et al. 45 probed the impact of partial loss of CTLA4 expression using heterozygous Ctla4-flox/wt mice expressing Foxp3-Cre. In both settings, increases in T FH cells were observed after immunization. Collectively, these findings illustrate that T reg cells are a non-redundant population mediating CTLA4-dependent regulation of T FH cells.

The above observations could potentially reflect the role of CTLA4 in follicular regulatory T (T FR ) cells, the subset of T reg cells that exhibit T FH cell features and enter GCs 44, 46, 49 . For T reg cells to acquire the T FR cell programme they downregulate CD25 (IL-2Rα) expression, permitting them to express BCL-6, which would otherwise be antagonized by IL-2-driven BLIMP1 expression 50, 51 . T FR cells are thought to optimize protective antibody responses while suppressing the generation of antibodies to self-antigens and allergens 44, 46, 49, [52] [53] [54] and, like T FH cells, T FR cells have a circulating counterpart that can be altered in autoimmunity 55, 56 ( fig. 1 ). Interestingly, selective depletion of T FR cells, either in mice with floxed Bcl6 and Foxp3-driven Cre expression 54, 57, 58 or in mice with STOP-floxed Cxcr5-DTR and Foxp3-Cre 53 , results in only minor or transient increases in T FH cells despite marked effects on B cell differentiation. As loss of CTLA4 expression in FOXP3 + cells increases T FH cells but loss of T FR cells does not, it is possible that substantial CTLA4 activity occurs outside of the B cell follicle during the initial encounter between T cells and DCs. At the T cell-B cell border and within the follicle, T FR cells may employ additional suppressive mechanisms such as expression of the neuropeptide neuritin 57 or targeting of B cell metabolic pathways 59 .

In a fascinating twist, it appears that FOXP3-based regulation of T FH cells can also operate in a cell-intrinsic manner. T FH cells themselves upregulate FOXP3 expression in late-stage GCs, and this is associated with loss of expression of the T cell help-associated genes IL21 and CD40L and GC collapse 60 . These FOXP3 + T FH cells express high levels of CTLA4 and are reminiscent of the CD25 -T FR cells described by Wing et al. 61 , the transcriptional profiles of which place them equidistant between T FH cells and activated T reg cells. The division of labour between T reg cells, T FR cells and FOXP3 + T FH cells will need to be dissected 155 , reducing the number of clones exposed to hypermutation. T cell-B cell interactions at the T cell-B cell border support the extrafollicular response 156 and dysregulation of extrafollicular T cells may lead to autoimmunity 157 . In the follicular response, B cells compete again for T cell help in the GC light zone, with those expressing the highest affinity receptors able to capture more antigen and present higher peptide concentrations. Immunoglobulin gene mutations resulting in enhanced antigen affinity trigger GC B cells to upregulate CCL22 and CCL17 expression, allowing them to preferentially attract T cell help 158 . Limiting the number of T cells available and ensuring that their cytokine output remains 'stingy' 159 are crucial features of the competitive environment required for affinity maturation to occur. by further experimentation. Taken together, T reg cell populations play a key role in controlling T FH cell numbers in both mice and humans, potentially via the CTLA4 pathway.

Many of the genes associated with susceptibility to autoimmunity are expressed in T reg cells 62 and the pre-eminent role for T reg cells in regulating autoimmunity is well recognized. Mice lacking T reg cells develop lethal autoimmunity 63 and humans with an impaired T reg cell compartment as a result of mutations in FOXP3 develop the aggressive early-onset immune dysregulation syndrome IPEX 64 . Interestingly, deficits in T reg cells can interfere with normal costimulatory control of T cell immunity -the unexpected exacerbation of disease in CD28-deficient non-obese diabetic mice was reconciled by the discovery of the role of CD28 in T reg cell development 65 , and recent findings suggest CD28 also contributes to T reg cell homeostasis in humans 66 . A replete T reg cell compartment is therefore key to the normal regulation of immune responses, and strategies aimed at augmenting T reg cell numbers, by low-dose IL-2 treatment or T reg cell therapy, are being actively pursued in settings of autoimmunity.

The IL-2 pathway is recognized as a major regulator of T FH cell differentiation ( fig. 2 ). In mice, exogenous provision of IL-2 has been shown to suppress T FH cell differentiation both in the context of viral infection 67 and autoimmunity 68 . In humans, IL-2 is also a known regulator of T FH cell differentiation 69 , and low-dose IL-2 therapy can decrease numbers of cT FH cells in individuals with autoimmune disease 70 .

One potential mechanism by which IL-2 inhibits T FH cells is by enhancing T reg cell homeostasis as IL-2 is a key cytokine for T reg cell expansion and maintenance [71] [72] [73] . However, it is clear that IL-2 can also act directly on conventional T cells to inhibit T FH cell differentiation. During T cell priming, IL-2 signalling can alter the balance between T FH cell and non-T FH cell effector differentiation 74,75 via STAT5-mediated skewing of the BLIMP1 to BCL-6 ratio 76, 77 .

In an interesting twist, T cells induced to make IL-2 are not those that respond to it -instead, the producers become T FH cells while using IL-2 in a paracrine manner to preclude their neighbours from this fate 78 . This finding establishes a biological role for the synaptic-based delivery of IL-2 between adjacent T cells undergoing activation 79 .

Such is the threat to T FH cell differentiation posed by IL-2 that a variety of mechanisms exists to subvert this inhibition. Transforming growth factor-β (TGFβ) insulates T cells from IL-2 signals by suppressing CD25 (IL-2Rα) expression 80 , which may contribute to its ability to promote T FH cell differentiation 81 . Activated DCs in the outer T cell zone upregulate CD25, allowing developing T FH cells to approach the B cell follicle in an environment quenched of local IL-2 (ref. 82 ). Mature T FH cells can be shielded from IL-2 in a different manner, becoming desensitized to IL-2 signalling via IL-6-mediated downregulation of the IL-2 receptor subunit CD122 (IL-2Rβ) 83 . The local availability of IL-2 and other cytokines is therefore an important factor in controlling T FH cell numbers. Intriguingly, in conditions of limiting IL-2, T helper 1 cells can increase their BCL-6 to T-bet ratio and assume a partial T FH cell phenotype 84 . Thus, some cells bearing T FH cell markers in autoimmune settings could conceivably have arisen via this route.

The intersection between IL-2-mediated and T reg cell-mediated control of T FH cell differentiation is a complex one. T reg cells might be expected to promote T FH cell differentiation by serving as an IL-2 sink 85,86 but they can also limit T FH cell formation by CTLA4-dependent regulation of costimulation 29, 45, 48 . One way to view this is that avoiding IL-2 signals may be necessary but not sufficient for T FH cell differentiation and, as highlighted above, this can be achieved through a variety of means. By contrast, there seems to be little redundancy in control of CD28 costimulation, with CTLA4-expressing T reg cells being required to restrict T FH cell numbers 45, 48 . Thus, in the absence of T reg cells, the effects of dysregulated CD28 costimulation may dominate over the lack of IL-2 consumption as other populations can compensate for the latter.

The IL-2 pathway is strongly implicated in susceptibility to autoimmune disease. Single nucleotide polymorphisms (SNPs) in IL2RA are associated with multiple autoimmune diseases, including T1D, RA, IBD, MS, Crohn's disease, alopecia areata and vitiligo (see GWAS catalogue). In addition to Il2RA SNPs 87 , autoimmune-associated variants have been identified in IL2, IL2RB and

Cytotoxic T lymphocyte antigen 4 (CTLA4) acts as a competitive inhibitor for the T cell costimulator CD28 as both receptors bind to the same ligands, CD80 and CD86, but CTLA4 binds with higher affinity. Furthermore, CTLA4 and CD80 form high-avidity dimer-dimer interactions 160 . CTLA4 can function in a cell-extrinsic manner to remove CD80 and CD86 from antigen-presenting cells by a process known as transendocytosis 161 . Expression of CTLA4 by regulatory T (T reg ) cells is required to prevent fatal autoimmune disease in mice 162 , and T reg cells can use CTLA4 to control the levels of CD80 and CD86 on dendritic cells trafficking from peripheral tissues to lymph nodes 117 . As the biological role of CTLA4 is to regulate the CD28 pathway, phenotypes associated with CTLA4 deficiency are lost in the absence of their shared ligands 163 .

Follicular helper T (T FH ) cells are reciprocally regulated by CTLA4 and CD28. Mice lacking CTLA4 systemically or in T reg cells exhibit exaggerated T FH cell differentiation and this is recapitulated by injection of blocking anti-CTLA4 antibodies 29, 45, 48 (see figure, part a) . Humans with CTLA4 trafficking defects also exhibit increases in T FH cell numbers 34 . Conversely, soluble CTLA4 fusion proteins decrease T FH cell numbers in mice and humans 35, 36, 38 . Mice with wild-type CD28 expression have intact T FH cell development, and those with lower CD28 expression due to Cd28 gene heterozygosity show a reduced propensity for T FH cell development, whereas complete CD28 deficiency abrogates T FH cell development 29, 31 (see figure, part b) . PTPN2. PTPN2 is a phosphatase involved in many signalling pathways, including IL-2 receptor signalling. T cell-specific deficiency in Ptpn2 increased frequencies of T FH cells and GC B cells in non-obese diabetic mice and was associated with exacerbated diabetes 88 . It is likely that multiple pathways control IL-2 receptor signalling as defects are evident in T1D and MS even after controlling for IL2RA and PTPN2 genotypes 89 . Mice deficient in IL-2 or the IL-2 receptor subunits CD122 (IL-2Rβ) or CD25 (IL-2Rα) develop lethal autoimmunity [90] [91] [92] and mutations in IL-2Rβ cause life-threatening immune dysregulation in humans 93 . Importantly, although IL-2 pathway genes can clearly modulate autoimmunity via effects on T reg cell homeostasis, they can also act in conventional T cells: for example, a SNP at the Il2RA locus associated with protection from T1D and MS was linked to higher levels of CD25 expression on conventional memory CD4 + T cells 94 . Consistent with the role of the IL-2 pathway in the regulation of autoimmunity, low-dose IL-2 can be used therapeutically across a wide range of autoimmune diseases 95 .

Collectively, these studies highlight the connection between control of T FH cell differentiation and the genetic regulation of autoimmunity. Notably, genes in the CTLA4 and IL-2 pathways are consistently highlighted in genetic analyses across a broad range of common autoimmune conditions 96, 97 , and SNPs associated with autoimmunity are enriched in CpG demethylated regions specifically found in T reg cells 62 .

This places CTLA4, IL-2 and T reg cells at the heart of the shared genetic susceptibility to autoimmune disease that underpins heritability. Importantly, any defects in the CTLA4 or IL-2 pathways or deficits in T reg cell homeostasis or function would lead to a dysregulated T FH cell compartment as well as to the propensity to autoimmune disease ( fig. 3 ). This could be one reason for increases in cT FH cells being seen across multiple autoimmune disease settings.

The principal biological role for T FH cells lies in protection from infectious disease. T FH cell differentiation in response to viral, bacterial, parasitic or fungal antigens is key for the generation of protective antibody responses, in particular affinity-matured neutralizing antibodies 98 . Mimicking infection by vaccination also induces T FH cells, with transient increases in cT FH cells being detectable in the blood 99 .

Notably, infectious triggers have been postulated for many autoimmune diseases [100] [101] [102] [103] [104] . Lyme disease is the quintessential example whereby immune responses to tick-borne Borrelia burgdorferi can give rise to Lyme arthritis with autoimmune T cell and B cell responses 105 . In most of the common autoimmune diseases, a clear link to an individual pathogen is lacking; however, circumstantial evidence is often strong. For example, there is a known association between enteroviral infections and T1D 106 , and enteroviral capsid protein 107 and an antiviral signature 108 have been detected in the pancreatic islets of people with T1D. Furthermore, recent data show a clear link between Epstein-Barr virus infection and the development of MS 109, 110 .

Although T FH cell increases are short-lived in acute infection, chronic viral infections elicit persistent T FH cell responses 111, 112 . Such increases in T FH cells could interfere with competitive selection within the GC, potentially allowing the survival of self-reactive B cells. Indeed, persistent viral infection can elicit polyclonal B cell activation and production of autoantibodies 113 . In light of this, it is interesting that prolonged enteroviral infection, rather than multiple short-duration infections, is associated with islet autoimmunity in T1D 114 .

The ability of self-reactive B cells to take up viral antigens, via pinocytosis, Fc receptors or complement receptors, may allow them to present viral peptides and solicit help from virus-specific T FH cells 113 . In experimental systems, B cells specific for the central nervous system self-antigen myelin oligodendrocyte glycoprotein (MOG) can co-capture influenza virus haemagglutinin and MOG from cell membranes and obtain help from haemagglutinin-specific T cells to produce anti-MOG antibodies 115 . Interestingly, the ability of T FH cells to provide help to bystander B cells and elicit autoantibody production appears to be enhanced by lymphopenia 20 , a state long associated with autoimmunity. Conceivably, lymphopenia in the context of SARS-CoV-2 infection could contribute to the generation of autoantibodies documented in patients with COVID-19 (ref. 116 ).

Chronic infections may alter T cell activation thresholds via the upregulation of costimulatory ligands in response to Toll-like receptor engagement or pro-inflammatory cytokines. This costimulatory ligand upregulation may outpace CTLA4-dependent ligand downregulation, permitting the activation of self-reactive T cells normally censored by insufficient CD28 engagement 117 . Prolonged increases in T FH cells could exacerbate this by increasing levels of IL-21, the archetypal T FH cell-associated cytokine, which can counteract T reg cell suppression 118, 119 and may reinforce T FH cell differentiation 120 . IL-21 is overexpressed in several autoimmune diseases, including SLE, RA, SS and T1D 121 , and although it can also derive from other cells, significant correlations between IL-21 production and T FH cell frequencies have been noted 122, 123 . Thus, chronic infectious settings associated with elevated IL-21, blunted T reg cell function and increased costimulation could permit self-reactive T cells to acquire a T FH cell phenotype.

Infection may play a role in epitope spreading, which is a feature of many autoimmune diseases and is frequently associated with disease progression. One mechanism that may contribute to this phenomenon is the invasion and reuse of existing GCs by B cells bearing a different specificity, particularly in the context of shared T cell help (for example, when B cells specific for distinct regions of a protein interact with the same T FH cell owing to presentation of a common peptide) 124 . Cross-reactive recognition of bacterial antigens by self-reactive T cells could conceivably form the basis of shared T cell help in autoimmune settings 125 . Notably, the presence of adjuvants, including the bacterial molecule lipopolysaccharide, can enhance GC invasion and reuse 124 , suggesting that, in addition to unleashing new cohorts of T FH cells, infection may also enhance the sharing of T cell help between B cells of different specificities.

Infection and autoimmunity may also intersect mechanistically at involvement of the type 1 interferon pathway. Type 1 interferons provide the key 'early warning' signal of viral infection and play complex roles in protection or pathology following infection with viruses, bacteria, parasites and fungi 126 . The type 1 interferon pathway has been widely associated with autoimmune diseases, most notably in the case of SLE but also in T1D, MS, RA and others [127] [128] [129] [130] . There are conflicting reports on the impact of type 1 interferons on T FH cell differentiation. There is evidence that type 1 interferons promote acquisition of at least some aspects of the T FH cell phenotype 131 , although other data suggest that they support T helper 1 cells at the expense of T FH cells 132 . Studies focusing on DCs as the target cells for type 1 interferons point to a positive role in T FH cell differentiation following immunization www.nature.com/nri or vaccination 133,134 but a negative role in the setting of Plasmodium infection 135 . As might be expected for the type 1 interferon pathway, timing, location and context are likely to be key in determining outcome 136 .

Different autoimmune diseases are highly distinct in their presentation, with unique tissue-specific features: for example, bone erosion in RA, renal pathology in SLE and metabolic abnormalities in T1D. Given the striking differences in disease processes and clinical presentation, it is remarkable that increases in cT FH cells appear to be a unifying theme across a large number of autoimmune diseases. In this article, I highlight that CTLA4, IL-2 and T reg cells are key modulators of T FH cell differentiation and are also major players in the regulation of autoimmunity. This fundamental connection may go some way to explaining why dysregulated T FH cell homeostasis is a feature of so many different autoimmune diseases. Reinforcing this connection, several other T FH cell-associated genes (including CXCR5, CCR7, ICOSL, PD1, IL4R, IL21R and CD40) are linked to autoimmune disease susceptibility. The notion that genetic predisposition to autoimmunity may be coupled to genetic predisposition to T FH cell formation provides a new perspective on the cT FH cell changes widely reported in autoimmune settings.

As infection can lead to T FH cell differentiation, it is conceivable that infectious triggers occurring on autoimmune-susceptible backgrounds may cause discernible changes to T FH cell populations that precede autoimmunity. Thus, T FH cells may represent the consummate biomarker in autoimmunity, neatly integrating genetic and environmental risk. Accordingly, T FH cell profiling may be increasingly important in the prediction of autoimmune disease development and in monitoring and predicting clinical response to immunotherapies 38 .

The precise contribution of T FH cells, and their circulating counterparts, to pathology likely differs between autoimmune diseases. Dysregulated T FH cell homeostasis may allow the production of autoantibodies that mediate diverse effector functions -for example, enhancing cross-presentation of islet antigens in T1D 137 , driving renal pathology in SLE 138 , and promoting osteoclastogenesis and bone loss in RA 139, 140 . As well as acting on B cells, the capacity for T FH cells to produce IL-21 and CXCL13 (ref. 8 ) may influence CD8 + T cell activation and ectopic lymphoid structure formation, respectively, whereas dysregulated IFNγ production by T FH cells 141 has been shown to drive autoimmune GCs and systemic autoimmune disease 142 .

Altogether, closer study of T FH cell populations in blood, lymphoid organs and tissues promises to yield new insight into autoimmune pathogenesis as well as allowing us to harness the full biomarker potential of this population.

Germinal centers

Follicular helper T cells

Pathophysiology of T follicular helper cells in humans and mice

A RING-type ubiquitin ligase family member required to repress follicular helper T cells and autoimmunity

SAP-controlled T-B cell interactions underlie germinal centre formation

Follicular helper T cells are required for systemic autoimmunity

Expansion of circulating T cells resembling follicular helper T cells is a fixed phenotype that identifies a subset of severe systemic lupus erythematosus

T follicular helper cells in human autoimmunity

Human blood CXCR5 + CD4 + T cells are counterparts of T follicular cells and contain specific subsets that differentially support antibody secretion

T-follicular helper cells survive as long-term memory cells

Bcl6 expressing follicular helper CD4 T cells are fate committed early and have the capacity to form memory

Distinct memory CD4 + T cells with commitment to T follicular helper-and T helper 1-cell lineages are generated after acute viral infection

Human circulating PD-1 + CXCR3 -CXCR5 + memory Tfh cells are highly functional and correlate with broadly neutralizing HIV antibody responses

Phenotype and functions of memory Tfh cells in human blood

ICOS deficiency is associated with a severe reduction of CXCR5 + CD4 germinal center Th cells

Monogenic mutations differentially affect the quantity and quality of T follicular helper cells in patients with human primary immunodeficiencies

Bcl6 protein expression shapes pre-germinal center B cell dynamics and follicular helper T cell heterogeneity

Circulating T follicular regulatory and helper cells have memory-like properties

Interconnected subsets of memory follicular helper T cells have different effector functions

Persistent antigen and germinal center B cells sustain T follicular helper cell responses and phenotype

T follicular helper cell dynamics in germinal centers

Circulating precursor CCR7 lo PD-1 hi CXCR5 + CD4 + T cells indicate Tfh cell activity and promote antibody responses upon antigen reexposure

Mapping the diversity of follicular helper T cells in human blood and tonsils using high-dimensional mass cytometry analysis

Vaccination establishes clonal relatives of germinal center T cells in the blood of humans

The adjuvant GLA-SE promotes human Tfh cell expansion and emergence of public TCRbeta clonotypes

T follicular helper cells in human efferent lymph retain lymphoid characteristics

Fingolimod profoundly reduces frequencies and alters subset composition of circulating T follicular helper cells in multiple sclerosis patients

Predominant autoantibody production by early human B cell precursors

CTLA-4 controls follicular helper T-cell differentiation by regulating the strength of CD28 engagement

CD28 is required for germinal center formation

Compromised OX40 function in CD28-deficient mice is linked with failure to develop CXCR5-positive CD4 cells and germinal centers

Phosphoinositide 3-kinase activity in T cells regulates the magnitude of the germinal center reaction

The microRNA cluster miR-17 approximately 92 promotes TFH cell differentiation and represses subset-inappropriate gene expression

Exaggerated follicular helper T-cell responses in patients with LRBA deficiency caused by failure of CTLA4-mediated regulation

Attenuation of follicular helper T cell-dependent B cell hyperactivity by abatacept treatment in primary Sjogren's syndrome

Circulating follicular helper T cells (CD4 + CXCR5 + ICOS + ) decrease in patients with rheumatoid arthritis treated with abatacept

Abatacept targets T follicular helper and regulatory T cells, disrupting molecular pathways that regulate their proliferation and maintenance

Follicular helper T cell profiles predict response to costimulation blockade in type 1 diabetes

Loss of CTLA-4 leads to massive lymphoproliferation and fatal multiorgan tissue destruction, revealing a critical negative regulatory role of CTLA-4

Lymphoproliferative disorders with early lethality in mice deficient in Ctla-4

Autosomal dominant immune dysregulation syndrome in humans with CTLA4 mutations

Immune dysregulation in human subjects with heterozygous germline mutations in CTLA4

Patients with LRBA deficiency show CTLA4 loss and immune dysregulation responsive to abatacept therapy

Follicular regulatory T cells expressing Foxp3 and Bcl-6 suppress germinal center reactions

Regulatory T cells control antigen-specific expansion of Tfh cell number and humoral immune responses via the coreceptor CTLA-4

Foxp3 + follicular regulatory T cells control the germinal center response

Accumulation of peripheral autoreactive B cells in the absence of functional human regulatory T cells

The coinhibitory receptor CTLA-4 controls B cell responses by modulating T follicular helper, T follicular regulatory, and T regulatory cells

Regulation of the germinal center reaction by Foxp3 + follicular regulatory T cells

Dynamic regulation of T follicular regulatory cell responses by interleukin 2 during influenza infection

Tfr cells lack IL-2Ralpha but express decoy IL-1R2 and IL-1Ra and suppress the IL-1-dependent activation of Tfh cells

Deficiency in T follicular regulatory cells promotes autoimmunity

Follicular regulatory T cells control humoral and allergic immunity by restraining early B cell responses

CD4 + follicular regulatory T cells optimize the influenza virus-specific B cell response

Control of germinal center responses by T-follicular regulatory cells

T follicular helper cells and T follicular regulatory cells in rheumatic diseases

Follicular regulatory T cells produce neuritin to regulate B cells

Follicular regulatory T cells repress cytokine production by follicular helper T cells and optimize IgG responses in mice

Suppression by TFR cells leads to durable and selective inhibition of B cell effector function

Expression of Foxp3 by T follicular helper cells in end-stage germinal centers

A distinct subpopulation of CD25(-) T-follicular regulatory cells localizes in the germinal centers

Regulatory T cell-specific epigenomic region variants are a key determinant of susceptibility to common autoimmune diseases

Disruption of a new forkhead/ winged-helix protein, scurfin, results in the fatal lymphoproliferative disorder of the scurfy mouse

Clinical and molecular features of the immunodysregulation, polyendocrinopathy, enteropathy, X linked (IPEX) syndrome

B7/CD28 costimulation is essential for the homeostasis of the CD4 + CD25 + immunoregulatory T cells that control autoimmune diabetes

Humans with inherited T cell CD28 deficiency are susceptible to skin papillomaviruses but are otherwise healthy

Interleukin-2 inhibits germinal center formation by limiting T follicular helper cell differentiation

Persistent IL-2 receptor signaling by IL-2/CD25 fusion protein controls diabetes in NOD mice by multiple mechanisms

Activin A programs the differentiation of human TFH cells

Low-dose interleukin-2 treatment selectively modulates CD4 + T cell subsets in patients with systemic lupus erythematosus

Homeostatic maintenance of natural Foxp3 + CD25 + CD4 + regulatory T cells by interleukin (IL)-2 and induction of autoimmune disease by IL-2 neutralization

A function for interleukin 2 in Foxp3-expressing regulatory T cells

In vivo expansion of T reg cells with IL-2-mAb complexes: induction of resistance to EAE and long-term acceptance of islet allografts without immunosuppression

ICOS receptor instructs T follicular helper cell versus effector cell differentiation via induction of the transcriptional repressor Bcl6

Opposing signals from the Bcl6 transcription factor and the interleukin-2 receptor generate T helper 1 central and effector memory cells

STAT5 is a potent negative regulator of TFH cell differentiation

STAT5 protein negatively regulates T follicular helper (Tfh) cell generation and function

Differential IL-2 expression defines developmental fates of follicular versus nonfollicular helper T cells

A synaptic basis for paracrine interleukin-2 signaling during homotypic T cell interaction

The transforming growth factor beta signaling pathway is critical for the formation of CD4 T follicular helper cells and isotype-switched antibody responses in the lung mucosa

The cytokine TGF-beta co-opts signaling via STAT3-STAT4 to promote the differentiation of human TFH cells

EBI2 augments Tfh cell fate by promoting interaction with IL-2-quenching dendritic cells

Inhibition of IL-2 responsiveness by IL-6 is required for the generation of GC-TFH cells

Molecular mechanisms that control the expression and activity of Bcl-6 in TH1 cells to regulate flexibility with a TFH-like gene profile

CD4 + CD25 + Foxp3 + regulatory T cells induce cytokine deprivation-mediated apoptosis of effector CD4 + T cells

An essential role for the IL-2 receptor in Treg cell function

Large-scale genetic fine mapping and genotype-phenotype associations implicate polymorphism in the IL2RA region in type 1 diabetes

T-cell-specific PTPN2 deficiency in NOD mice accelerates the development of type 1 diabetes and autoimmune comorbidities

Multiple autoimmune-associated variants confer decreased IL-2R signaling in CD4 + CD25 hi T cells of type 1 diabetic and multiple sclerosis patients

Ulcerative colitis-like disease in mice with a disrupted interleukin-2 gene

Deregulated T cell activation and autoimmunity in mice lacking interleukin-2 receptor beta

Interleukin-2 receptor alpha chain regulates the size and content of the peripheral lymphoid compartment

Human interleukin-2 receptor beta mutations associated with defects in immunity and peripheral tolerance

Cell-specific protein phenotypes for the autoimmune locus IL2RA using a genotype-selectable human bioresource

Immunological and clinical effects of low-dose interleukin-2 across 11 autoimmune diseases in a single, open clinical trial

Genetic and epigenetic fine mapping of causal autoimmune disease variants

Pervasive sharing of genetic effects in autoimmune disease

T follicular helper cell biology: a decade of discovery and diseases

Induction of ICOS + CXCR3 + CXCR5 + TH cells correlates with antibody responses to influenza vaccination

Triggers of autoimmunity: the role of bacterial infections in the extracellular exposure of lupus nuclear autoantigens

Autoimmunity to nucleosomes related to viral infection: a focus on hapten-carrier complex formation

Enteroviral infections as a trigger for type 1 diabetes

Aggregatibacter actinomycetemcomitans-induced hypercitrullination links periodontal infection to autoimmunity in rheumatoid arthritis

Infection as an environmental trigger of multiple sclerosis disease exacerbation

Lyme arthritis: linking infection, inflammation and autoimmunity

Association between enterovirus infection and type 1 diabetes risk: a meta-analysis of 38 case-control studies

Expression of the enteroviral capsid protein VP1 in the islet cells of patients with type 1 diabetes is associated with induction of protein kinase R and downregulation of Mcl-1

Islet expression of type I interferon response sensors is associated with immune infiltration and viral infection in type 1 diabetes

Longitudinal analysis reveals high prevalence of Epstein-Barr virus associated with multiple sclerosis

Clonally expanded B cells in multiple sclerosis bind EBV EBNA1 and GlialCAM

Sustained T follicular helper cell response is essential for control of chronic viral infection

Viral persistence redirects CD4 T cell differentiation toward T follicular helper cells

Hypergammaglobulinemia and autoantibody induction mechanisms in viral infections

Prospective virome analyses in young children at increased genetic risk for type 1 diabetes

Cocapture of cognate and bystander antigens can activate autoreactive B cells

Diverse functional autoantibodies in patients with COVID-19

CTLA-4-mediated transendocytosis of costimulatory molecules primarily targets migratory dendritic cells

IL-21 counteracts the regulatory T cell-mediated suppression of human CD4+ T lymphocytes

Release from regulatory T cell-mediated suppression during the onset of tissue-specific autoimmunity is associated with elevated IL-21

A fundamental role for interleukin-21 in the generation of T follicular helper cells

Interleukin-21: a double-edged sword with therapeutic potential

Follicular helper T cell signature in type 1 diabetes

IL-21 production by CD4 effector T cells and frequency of circulating follicular helper T cells are increased in type 1 diabetes patients

Germinal center reutilization by newly activated B cells

Nature of T cell epitopes in lupus antigens and HLA-DR determines autoantibody initiation and diversification

Type I interferons in infectious disease

Interferon and granulopoiesis signatures in systemic lupus erythematosus blood

Type I interferons: crucial participants in disease amplification in autoimmunity

A type I interferon transcriptional signature precedes autoimmunity in children genetically at risk for type 1 diabetes

A subtype of multiple sclerosis defined by an activated immune defense program

Type I IFN induces binding of STAT1 to Bcl6: divergent roles of STAT family transcription factors in the T follicular helper cell genetic program

Transcription factor STAT3 and type I interferons are corepressive insulators for differentiation of follicular helper and T helper 1 cells

Type I interferon signaling in dendritic cells stimulates the development of lymph-node-resident T follicular helper cells

Rejuvenating conventional dendritic cells and T follicular helper cell formation after vaccination

IFNAR1-signalling obstructs ICOS-mediated humoral immunity during non-lethal blood-stage plasmodium infection

Spatiotemporal regulation of type I interferon expression determines the antiviral polarization of CD4 + T cells

Antibody-enhanced cross-presentation of self antigen breaks T cell tolerance

Autoantibodies in systemic autoimmune diseases: specificity and pathogenicity

Induction of osteoclastogenesis and bone loss by human autoantibodies against citrullinated vimentin

Periarticular bone loss in arthritis is induced by autoantibodies against citrullinated vimentin

P2X7 receptor restrains pathogenic Tfh cell generation in systemic lupus erythematosus

B cell IFN-gamma receptor signaling promotes autoimmune germinal centers via cell-intrinsic induction of BCL-6

The transcriptional repressor Bcl-6 directs T follicular helper cell lineage commitment

Bcl6 and Blimp-1 are reciprocal and antagonistic regulators of T follicular helper cell differentiation

Bcl6 mediates the development of T follicular helper cells

Bcl-6 is the nexus transcription factor of T follicular helper cells via repressor-of-repressor circuits

The costimulatory molecule ICOS regulates the expression of c-Maf and IL-21 in the development of follicular T helper cells and TH-17 cells

Batf coordinates multiple aspects of B and T cell function required for normal antibody responses

The transcription factor BATF controls the global regulators of class-switch recombination in both B cells and T cells

Transcription factor achaete-scute homologue 2 initiates follicular T-helper-cell development

ICOS maintains the T follicular helper cell phenotype by down-regulating Kruppel-like factor 2

Functional STAT3 deficiency compromises the generation of human T follicular helper cells

Cutting edge: STAT1 is required for IL-6-mediated Bcl6 induction for early follicular helper cell differentiation

Antigen-engaged B cells undergo chemotaxis toward the T zone and form motile conjugates with helper T cells

A dynamic T cell-limited checkpoint regulates affinity-dependent B cell entry into the germinal center

Human tonsil B-cell lymphoma 6 (BCL6)-expressing CD4 + T-cell subset specialized for B-cell help outside germinal centers

ICOS-dependent extrafollicular helper T cells elicit IgG production via IL-21 in systemic autoimmunity

Affinity-coupled CCL22 promotes positive selection in germinal centres

A cytokine-independent approach to identify antigen-specific human germinal center T follicular helper cells and rare antigen-specific CD4 + T cells in blood

The interaction properties of costimulatory molecules revisited

Trans-endocytosis of CD80 and CD86: a molecular basis for the cell-extrinsic function of CTLA-4

CTLA-4 control over Foxp3 + regulatory T cell function

B7-1 or B7-2 is required to produce the lymphoproliferative phenotype in mice lacking cytotoxic T lymphocyte

Human blood Tfr cells are indicators of ongoing humoral activity not fully licensed with suppressive function

The author is an inventor on a patent relating to T FH cell profiles and predicting response to costimulation blockade in autoimmunity.

Nature Reviews Immunology thanks the anonymous reviewers for their contribution to the peer review of this work.