key: cord-007149-m4xsx9ev authors: Morahan, Pages S; Connor, Janice R; Leary, Kathryn R title: VIRUSES AND THE VERSATILE MACROPHAGE date: 1985-01-17 journal: Br Med Bull DOI: 10.1093/oxfordjournals.bmb.a072017 sha: doc_id: 7149 cord_uid: m4xsx9ev Mononuclear phagocytes, including circulating monocytes and tissue macrophages, play a central role in resistance to viruses. This resitance can be expressed both non-specifically, and specifically in indiction, regulation and amplification of humoral and cell mediated immune responses to viruses. These lead to the extrinsic effect of macrophages on other virus-infected cells or free virus, and the intrinsic effect on viruses within macrophages. While these interactions usually appear to be protective, immunopathologic consequences as well as macrophage dysfunctions have also been noted. The outcome of any given interaction (viral elimination, peristance, latency or transformation) varies markedly with the type of macrophage. The molecular mechanisms involved in these very diverse macrophage-virus interactions are currently under study. Mononuclear phagocytes, including circulating monocytes and tissue macrophages, play a central role in resistance to viruses. This resistance can be expressed both non-specifically, and specifically in induction, regulation and amplification of humoral and cell mediated immune responses to viruses. These lead to the extrinsic effect of macrophages on other virus-infected cells or free virus, and the intrinsic effect on viruses within macrophages. While these interactions usually appear to be protective, immunopathologic consequences as well as macrophage dysfunctions have also been noted. The outcome of any given interaction (viral elimination, persistence, latency or transformation) varies markedly with the type of macrophage. The molecular mechanisms involved in these very diverse macrophage-virus interactions are currently under study. Mononuclear phagocytes (MP) comprise a widely distributed cell system that includes immature cells in the bone marrow, circulating monocytes and tissue macrophages (M). M are placed strategically throughout the body to meet foreign particles, and are prominent in the liver, lung, spleen and body cavities. The importance of MP in non-specific resistance to viruses was emphasized first in the 1960s. 1 " 3 Now it is recognized that they also play a central role in the induction, regulation and amplification of specific immune responses, as well as being pivotal in nonspecific resistance, homeostasis and synthesis of potent biologic mediators. The importance of MP perhaps can be appreciated best by their very early phylogenetic appearance, and the fact that no genetic defect resulting in absence of MP has yet been found in vertebrates that is compatible with life. The reader is referred to several recent reviews of the role of MP in resistance to herpes simplex virus (HSV) and other viruses for most work published prior to 1982. 4~* Extensive data document the importance of MP in resistance to virus infections in animal models, and probably in human disease (Table I) . 4 " 8 It has been difficult to deplete MP selectively in order to determine their precise role, but a new experimental system may prove to be useful-treatment of mice with radioactive 89 Sr to destroy bone marrow and thus remove marrow dependent cells. We have found that 89 Sr causes a rapid and profound decrease in circulating monocytes, polymorphonuclear leukocytes and in NK cell activity in CD1 mice, but does not have significant effects on the number or function of tissue M4>. 9 Moreover, natural or immunomodulatorenhanced resistance to encephalomyocarditis (EMC) virus 10 and HSV (unpublished observations) was not markedly changed from that in non-treated mice. These data provide evidence for a prominent role for tissue M<}> in non-specific resistance to these viruses. It is becoming increasingly apparent that MP do not act in isolation. There is a dynamic and balanced interplay of MP with other resistance mechanisms, most notably natural killer cells and specific cell mediated and humoral immune responses (see pp. 22-27 & 92-97) . 61 '-' 4 Delineating the relative roles for the various immune elements in vivo is a major research goal. In vitro systems have provided experimental approaches to define these elements in isolation and in controlled interactions. Extrinsic resistance of MP is defined as their ability to inactivate extracellular virus or reduce production in other surrounding cells that are normally permissive. The intrinsic interaction is defined as the permissiveness/non-permissiveness of the MP itself for growth of a virus. The extrinsic ability of MP to suppress virus production in another cell is independent of the ability of the MP to support virus replication itself. 615 The interactions may be completely non-specific, or may be modified immunologically in either direction. The extrinsic interaction may be modified by specifically immune cells, antibody, complement or lymphokines. T lymphocytes may activate MP for increased antiviral activity, thus amplifying specific T cell immunity. • 6 The activity of low levels of specific antibody can be amplified by MP-mediated antibody dependent cellular cytotoxicity (ADCC) for virus infected cells. 14 ' 7 When primary cultures of MP are used, it can be difficult to isolate the effects of MP alone from combined effects with these other immunologic elements or contaminating lymphoid cells.' 8 "' 9 Unfortunately, there have been few studies of the extrinsic interaction of pure cultures of bone-marrow-derived M (BMDMcfi), M clones or M-like cell lines with virus infected cells. Such studies will be invaluable in defining at the molecular level the mechanisms involved in MP mediated extrinsic antiviral activity. Viruses that have been shown to be inhibited by extrinsic MP effects include HSV, cytomegalovirus (CMV), Marek's disease, mouse hepatitis (MHV), ectromelia, EMC and vesicular stomatitis viruses (VSV). 4 ' 6 -15 -20 " 22 Several operative mechanisms have been demonstrated (Table 2) . Undoubtedly multiple processes are involved in limiting such diverse virus groups, e.g., if a virus has a high requirement for arginine, then arginine depletion by arginase secreted by MP may be important. 23 The mechanism in any given interaction appears to differ depending upon the particular MP, and the particular 'Achilles heel' of the virus replication strategy in the permissive cell. It is not yet clear whether diverse functional Table 1 Evidence for involvement of mononuclear cells in resistance to viruses in experimental animal models MonocytM and M4> predominate at site* of viral infection M* transfer resistance, particularly in neonatal annuls which form a model of M+ deficiency M4 often required for T cell or antibody transfer of resistance Mi> activated during virus infection for increased antiviral activity M+ activation by invnunomodutators. although other cells (e.g. NK cells) may also be activated, increases resistance M4 depletion may decrease resistance Increased M* antiviral action (e g, age, genetic factors) often correlates with increased resistance Table 2 Reported mechanisms of extrinsic mononuclear phagocyte-mediated antiviral activity In MP-virus intrinsic interactions there are also a multitude of possible outcomes. These include abortive, cytolytic, persistent non-cytolytic, and latent infections, and transformation. The nonpermissiveness of MP for most viruses has been the subject of numerous investigations. 4 -6 -24 -25 -113 MP-mediated oxidative metabolism, which is important in some tumoricidal and microbicidal activities, was not important for intrinsic resistance to VSV infection in parental or oxidative metabolism-defective cloned MaVlike cell lines. 21 A possible role for MP secretion of arginase has also been studied, but conflicting results have been obtained. 2326 -27 Interferons produced by MP appear to play a complex role in intrinsic resistance. Interferon had no effect on African swine fever virus, 28 but decreased production of rubella virus in MP. 29 Interferon induced by a high multiplicity of infection with HSV was correlated with inhibition of productive virus replication in human monocytes. 30 The authors suggested that interferon caused antiviral activity and also inhibition of monocyte to M differentiation in vitro. Whether the endogenous interferon production was related to defective interfering particles remains to be determined. The use of anti-interferon serum treatment in vivo has suggested that endogenous interferon may be present under some physiologic conditions and maintain resident peritoneal M (ResM(J>) in a nonpermissive state for certain viruses. 31 ' 18 The recent finding that human MP infected with different viruses produce different alpha interferons with distinctive antiviral spectra is also of interest. 32 The interaction of alpha/beta interferon, but not gamma interferon, with the Mx gene for resistance to influenza virus in M4>, is an elegant example of the complex mechanisms governing the outcome of intrinsic M<(>-virus interactions "-119 MP are morphologically and functionally very diverse and the origins of this diversity have not yet been delineated. 34 Recent studies have established the effect of in-vitro 'ageing' and the maturation of monocytes to MfJ> in the M-virus interaction. Ageing in culture appears to increase replication of HSV, 26 " 30 caprine arthritis-encephalitis virus, 35 and rubella virus 29 in MP. What relationship ageing in vitro has to in-vivo differentiation/ maturation of MP is unresolved. 120 Cells susceptible to infection and transformation by avian myeloblastosis virus (AMV) have been found at all stages of MP differentiation. 36 Regardless of the stage of differentiation only a subset of Mr}> are targets for transformation by AMV. 37 The same has been described for productive infection with HSV, 6 CMV, 38 -39 lactic dehydrogenase (LDH), 40 -41 rubella 29 and influenza viruses. 42 Most laboratories have reported that only 3-20% of the MP are infected. With LDH, the M<$> permissiveness for virus has been both reported to be related to, 116 and not to be related to la antigen expression (Brinton, personal communication). The reasons for the apparent permissiveness and non-permissiveness within a M population remain an enigma. It is not known whether the heterogeneity is related to cell cycle, to stages of differentiation, to separate sublines of M<(>, or transient modulation by environmental factors. In addition to the heterogeneity that exists among MP naturally in vivo or through culture in vitro, MP can be altered by a myriad of treatments in vivo and in vitro. Immunization increases intrinsic resistance of M<}> to some viruses, 6 -43 but not others. 24 - 55 Intrinsic resistance of M to HSV was reported to be decreased in rats bearing a transplantable epithelioma. 44 Treatment of the MP in vitro with cytochalasin B or phytohaemagglutinin has been reported to alter permissiveness. 29 - 45 The use of agents to elicit tissue M, however, is probably a major source of variability in virus-M interactions observed among different laboratories, and this must be recognized when comparing various systems of virus-M(j> interactions. 6 An interferon-mediated mechanism in M for the immunomodulator, Corynebactenum parvum (CP), has been reported recently for ectromelia virus, 20 but does not appear to be effective for MHV. In general the attempts to use agents such as CP, interferon, or lipopolysaccharide, 24 to elucidate the mechanisms for M intrinsic resistance, have provided more confusion than consensus. There have been few studies of possible changes in the intrinsic resistance of non-dividing MP when the cells are put under the proliferative stimulus of colony-stimulating factor. Proliferating BMDM4> appear to be more susceptible to guinea pig herpes-like virus, 48 and MHV. 49 Proliferation of variously elicited peritoneal M<(> resulted in increased production of LDH and Sindbis viruses, 47 but not of HSV. 50 Results with LDH suggest generation of new transiently permissive cells from non-permissive precursors (116 and Brinton, personal communication). We have found major differences between ResMcS and BMDM4> in their interactions with HSV (see later section). Whether elicitors and MP growth stimulators increase the mitotic index of tissue M4>, which is then responsible for increased viral infection, is an intriguing possibility. Efficiency of infection and transformation with AMV was directly related to the mitotic activity of MP. 36 The above highlights have pointed out the diversity in MP from the same body site, and how these cells are altered by various treatments. There is also heterogeneity in permissiveness of MP from different organs. For example, mouse alveolar and peritoneal M<) > are reported to respond very differently to infection with influenza virus, 42 while LDH appeared to replicate equally well in M from different organs. 41 The genetic background of the animal provides a final source of variation in the virus-McJ) response. In a few instances, e.g. MHV 51 and Rift Valley fever viruses (Rosebroeck and Peters, personal communication), the resistance of the animal to infection paralleled MP resistance to the virus. More commonly, the genetic background affects all cells in a relatively equal fashion. 52 In summary, MP constitute a very diverse population of cells. Compounding this natural heterogeneity with the imposed heterogeneity of experimental systems precludes drawing broad conclusions regarding mechanisms of Mf^-virus interactions (Table 3) . Where data are most intriguing, investigators have endeavoured to isolate and thoroughly characterize a single animal-virus system. 36 Likewise, where in-vivo infections are mimicked by invitro results, there is reassurance of the validity of the in-vitro observations. 40 Organ source of MP peritoneal, blood, bone marrow, lung, spleen, liver Antmal species Animal age neonatal, adult Genetic background Culture conditions in vitro medium, duration, adherent or suspension culture Treatment of animals in vivo elicitation with thiogrycollate broth, mineral oil, proteose peptone, activation with C parvxjm. BCG, immunization with live or killed viruses Treatment of M<$> in vitro lipopotysacchande, interferon, macrophage activating factor Mifc proliferation M4> subpopulations size, la antigens, ectoenzymes, Miji differentiation antigens defining the interactions of M and viruses. Monoclonal antibodies to MP have the potential to separate and identify subpopulations of MP heterogeneous in functional phenotypes, as well as tissue-types, and elicited and resident Mo>. 34 The studies of the effects of local microenvironment on expression of various markers and ectoenzyme profiles hold great promise" 4115 in determining the origins of functional variability of the MP branch of the immune network. As with lymphocyte responses, MP responses can either be beneficial or harmful. T lymphocyte-activated MP can eliminate virus and be useful in most extraneural sites, but are pathologic in the central nervous system (see pp. [75] [76] [77] [78] [79] Similarly, while MPmediated ADCC lyses virus infected cells and thus decreases virus, ADCC can also cause massive tissue necrosis. In acute infection of adult mice with CMV, activated M((> can clearly be protective. 4 ' 6 At the same time, it has been documented that splenic M(J> are a major site for productive CMV infection and, if mice are splenectomized, resistance to CMV is increased. 53 Other work has shown that only a small percent of the M are cytolytically infected, 54 and that M may be the predominant site for latent infection with CMV and guinea pig herpes-like virus. 48 -54 -55 Thus, CMV may replicate preferentially in a subpopulation of splenic M<( > and BMDMo), while other M<}> are relatively resistant. The role of latent viral infection in MP as a means of harbouring potentially activatable virus merits study with other viruses in addition to these two herpesviruses. Another type of pathologic interaction is exemplified by visnamaedi retrovirus infection in sheep and goats. 56 The virus produces a persistent non-cytolytic infection in monocytes and tissue M. Thus protected from immune responses, infectious virus is continually disseminated to cells permissive for cytolytic infection. A few other viruses also appear to have a predilection for persistent and often non-cytolytic infection in MP, a mechanism that may be related to infection chronicity (Table 4 ). In addition, the pathogenesis of scrapie has recently been suggested to involve replication of the agent in MP. 57 Considerable investigation has been conducted to define antibody enhancement of virus infection of MP. This phenomenon has been reported with a wide range of RNA viruses. 5S~63 Enhanced infection requires monocytes, peritoneal MuJ>or M(J>-like cell lines with Fc receptors, and non-cytophilic IgG antibody that exhibits virus serotype or cross-reactive specificity. 58 " 65 In one case, some enhancement was also observed with IgM and the CR3 complement receptor. 66 The effective virus multiplicity of infection is apparently increased. Increasing phagocytosis by treatment of M with activating agents also increased virus uptake and replication. 67 It is hypothesized that dengue virus hemorrhatgic fever, in individuals undergoing a second infection, involves non-neutralizing antibody which enhances virus infection in monocytes. The infected monocytes then become targets for T cell immune elimination and release inflammatory mediators. 68 " 69 MP can be extremely sensitive targets for histocompatibilityrestricted T cell cytotoxicity, presumably because of the prominent expression of major histocompatibility complex antigens on the MP surface." There is an obvious need to determine whether this phenomenon plays a general role in viral pathogenesis. This section briefly updates Mogensen's recent review. 70 Virus infected cells can produce chemotactic stimuli or inhibitors. 4 -70 " 72 Moreover, the intrinsic interaction of viruses with MP may alter MP behaviour (Table 5 ). Cytolytic infection of MP removes the MP system in a local or systemic manner. The subsequent decreased resistance can be demonstrated by frog virus 3 destruction of hepatic Kupffer M<}>, allowing hepatocytes to become available for vaccinia virus infection. 73 The more subtle outcomes of intrinsic interactions, i.e. abortive or persistent infections, can also alter MP functions (Table 4 ). Depressed function of alveolar M4> after infection of mice with CMV, Sendai, or influenza viruses has been associated with predisposition to secondary microbial infection in the lung 70 -74 Studies in vitro have focused on changes in MP receptors, phagocytosis, oxidative metabolism, phagosome-lysosome fusion and microbicidal killing. 70 -75 " 80 Usually depressed function has also been observed, although no effect and even enhanced activity has been noted These conflicting results may be related to different M<}> populations, to M4> differentiation, or to the particular virus used. Other MP activities may be affected by virus infection. Mice acutely or chronically infected with MHV or Sendai virus showed impaired wound healing, which could be overcome by local administration of M(J> stimulating agents. 81 Infection of MQJ> with Pichinde virus, 82 Newcastle disease or lymphocyte choriomeningitis viruses 70 inhibited M((> proliferation in response to colony stimulating factor. We have also observed that MHV infection changes the ectoenzyme phenotype of ResMaJ) to that of activated M<(>, and may inhibit proliferation of BMDM4> (Dempsey, unpublished observations). Few studies have addressed virus effects on MP immunoregulation, which may play a central role in viral pathogenesis and in immunopathologic responses. Monocytes or M4> infected with CMV, influenza, Sendai or poliovirus have been shown to be suppressive Functions of mononuclear phagocytes that may be affected by virus infection Chemotaxis Attachment and phagocytosis of particles through nonspecific, Fc or complement receptors Intracallular oxidatrve response Lysosome-phegosome fusion Intracellular microbicidaJ activity Synthesis and/or secretion of txologicalry active molecules, e.g. prostaglandini, neutral proteases, interferon and complement Antigen presentation Regulation of immune responses, i e accessory and suppressor activity M$ activation procees for microbicidai and tumoncidal activity Antibody dependent cytotoxicrty (ADCC) by M$ Wound healing ONA synthesis and M$ proliferation in response to macrophage growth factor Mi) differentiation for certain lymphocyte responses. 83 " 85 LDH infection has been reported to interfere with MP antigen presentation. 41 Bovine rhinotracheitis and HSV infections have been shown to inhibit MfJ) ADCC activity. 76 " 78 A recent report indicates that only early CMV protein synthesis, without any apparent morphologic change in the M, was required for M4> dysfunction. 86 " The relevance that such M()> immunoregulatory changes may have during viral infections requires further investigation. The preceeding sections have reviewed the extensive data available on the biological interactions of M4> and viruses. There is now a distinct need to detail the precise mechanisms at the molecular level. The heterogeneity of M is well known, but the gene expression involved in the evolution of this heterogeneity is poorly understood. The overall rate of RNA synthesis in polyinosinic-polycytidylic acid activated M<}>, elicited with proteose peptone, is decreased. 86b Concomittently, there is an increased rate of glucose oxidation and cytolytic activity and perhaps an increased rate of protein synthesis. This suggests a selective turn-on/off of specific gene products during M(J> activation. Recent work, using recombinant cDNA clones of mRNAs in activated RAW 264.7 M heterogeneity and differentiation. The elucidation of M((>-virus interactions is approaching adolescence. We are beginning to note the terms 'viral genomes', 'use of cloned probes', 'synthesis of mRNA transcripts' etc. For example, Haller et al. discovered that M bearing the Mx loci, upon stimulation with interferon, could no longer replicate influenza virus. 52 Virus penetration was normal, yet no viral proteins were synthesized. 87 When the cells were analyzed by RNA/RNA hybridization, normal influenza virus transcripts were present, which could not be translated. In a cell-free in-vitro translation system, virus-specific proteins were synthesized from these mRNAs. 33 A unique protein induced by interferon in Mx-bearing cells has been identified; 88 what selective action it may have on the translation of primary influenza virus transcripts remains undetermined. Selected restriction of viral replication by different populations of M has also been studied by molecular biology techniques. Interaction of HSV-1 with various mononuclear phagocytes and Vero celJs MCMV-infected TGM and ResM showed virus-specific DNA present in approximately 85% of each cell population, but more infectious centres in TGM<)>. 55 Moreover, when TGM(() or ResM from latently infected mice were examined, virus was produced only from the TGM even though there were low levels of MCMV DNA in both cell types. The authors concluded that activation of M playsa major role in the in-vivo activation of MCMV infection following latency. In another non-M4> cell line, HCMV also does not replicate unless the cells have been stimulated to differentiate. 89 Stevens and Cook have shown the presence of HSV DNA and empty virus particles in M unable to replicate HSV. 90 To examine the mechanism of restriction in more detail, we compared the replication of HSV-1 in BMDM, ResM and permissive Vero cells (Table 6 ). At 24 hour post infection (PI) in Vero cells, the cells showed considerable cytopathic effect (CPE) and a yield of HSV of about 10 7 PFU/culture (100 PFU/cell). The BMDMo> also showed marked CPE at 48 h PI but no evidence of production of infectious virus, the yield being 10 4 PFU/culture (0.03 PFU/cell). ResM4> showed no CPE and no evidence of infectious virus (10 2 PFU/culture, 0.0005 PFU/cell). Using cloned Eco Rl fragments of the HSV-1 genome 91 as radioactive probes in cell and dot-blot analysis, 60% of the HSV-1 genome (Eco Rl fragments D, G, N, F, M, O, A and I) was present in all cell types at 2h PI. In BMDM4>, viral DNA replication occurred; the number of viral genome equivalents per cell increased with time. In contrast, the viral DNA content in ResM4> decreased to undetectable levels by 48h PI. Eco Rl fragments H, L, EK, and JK representing the terminal repeats, joint region and part of unique long region are reported to contain sequences that cross-hybridize with cellular sequences (Sandri-Goldin personal communication). 9293 The examination of this region of the HSV-1 genome in infected MrJ> by the more detailed Southern blot analysis 94 showed results similar to those for the other parts of the HSV genome. These data provide evidence that the block in virus production in BMDM occurs at a point beyond viral DNA replication, while the block in ResMrJi is prior to HSV DNA synthesis. However, we do not know if a subpopulation of BMDM replicate HSV-1 DNA. While a definition of M4> heterogeneity in molecular terms will enable us to better understand M<$> differences and viral interactions, the development of M-like cell lines indicate that permissiveness for VSV and HSV is not solely dependent upon the ability of M to divide. J774.16 cells showed relative non-permissiveness for VSV, 21 similar to ResMcJ). 31 There is variable permissiveness for HSV among M<(>-like cell lines. 6 ' 50 WEHI-3 were as non-permissive as ResM4>, while mouse PU5-1.8 and human U937 cells were moderately permissive, although not as much as Vero cells. 6 As with non-dividing M, usually only a small percentage of any of these cells are productively infected. The metabolic changes related to cell cycle that may be involved in permissiveness/nonpermissiveness in cloned cell lines merits additional study. Several viruses, particularly MHV and LDH, appear to infect M preferentially in vivo, but their replication in M<}>-like cell lines is variable. Feline infectious peritonitis virus replicated well in the fewf feline M(J)-like cell line. 95 LDH has not been demonstrated to replicate in a large group of M4>-like cell lines (Brinton, personal communication). The replication of MHV in M4>-like cell lines has not been investigated in detail. Many of the M<}>-like cell lines currently available have evolved as a result of transformation by viruses. Whether MP are more or less permissive for viral transformation than other cells is not known. Simian virus40 (SV40) transforms many cell types including M. 96 The efficiency of transformation can be greatly increased by transfection with SV40 DNA containing a defective DNA replication origin (SV40 ori"). 97 Two murine M4> cell lines from BMDM or splenic Mifr and one human monocyte/M()> cell line have been developed by transfection with SV40 oriD NA. 98 ' 99 When a non-M cell line was transformed in this manner, its capacity to differentiate was not altered even though it actively expressed SV40 T antigen. 100 Whether all MP properties are also retained in such transformed cells should be carefully monitored. Nonetheless, in addition to the currently available cell lines, it should now be possible to generate a variety of new M<)>-cell lines at various stages of differentiation whose ability to permit virus replication can be studied in detail. Regulation of the control of virus replication can then be examined by inducing each of these cells to differentiate in vitro beyond their 'frozen' state. In addition to transformation of cells by SV40, the retroviruses have contributed to the generation of immortalized M-like cells.' °' However, there is now strong evidence to suggest that the cellular oncogenes (related to the transforming genes of the retroviruses) play a major role in the control of differentiation of M4> and other cells.' °2 Some evidence for this can be derived from temperature sensitive mutant studies on myeloproliferative sarcoma virus transformation of fibroblasts and haematopoietic cells. The same gene function(s) appear to be involved in the transformation of the two very different cell types. 103 In the haematopoietic precursor cell line 416B, the levels of endogenous cellular P21 sarc (immunologically related to the src gene product of Harvey murine sarcoma virus) may be related to differentiation. 104 In the promyelocytic cell line, HL60, regulation of cellular oncogenes is seen following induction of differentiation with dimethyl sulphoxide or retinoic acid. 105106 While regulation of differentiation by cellular oncogenes is not restricted to the MP lineage, it clearly plays an important role. With this knowledge, it may now be possible to predict and test for some of the steps involved in MP differentiation, following the examples set for non-MP cells. Other methods for the generation of M<(> cell lines include the formation of somatic cell hybrids 107 -108 or clones. 109 LDH has been reported to replicate transiently in a mouse M<(>-human fibroblast hybrid. 108 The recent work of Johnson et al. has successfully demonstrated that normal M may be cloned and propagated continuously without the complications of viral transformation and somatic cell hybridization. 109 Before the versatile MP system can be manipulated successfully for immunotherapy against virus infections, methods need to be established to increase MP resistance and decrease immunopathologic outcomes. We have reviewed much data concerning the biological interactions of viruses or virus-infected cells with MP, and the importance of the differentiated and/or activated state of that MP to the outcome of the interaction. A crucial issue now is delineation of the origins of diversity of these cells at the molecular level. Only then will the control of MP gene expression, which determines the differing degrees of extrinsic and intrinsic MP antiviral activity, become evident. The field is now primed for this type of detailed experimental analysis that should lead to potent immunotherapeutic manipulation of the MP system. Liggitt HD Susceptibility of blood-derived monocytes and macrophages to caprine arthritisencephalitis virus Target cells for avian myeloblastosis virus in embryonic yolk sac and relationship of cell differentiation to cell transformation Replicating, differentiated macrophages can serve as in vitro targets for transformation by avian myeloblastosis virus Comparison oftechniques for recovering murine cytomegalovirus from a macrophage-enriched subpopulation of mice Murine peritoneal macrophages support murine cytomegalovirus replication Replication of lactate dehydrogenase-elevating virus in macrophages. 2. Mechanism of persistent infection in mice and cell culture Acute infection of mice with lactic dehydrogenase virus (LDV) impairs the antigen-presenting capacity of their macrophages Interaction of influenza virus with mouse macrophages In vitro infection of murine macrophages with junin virus Tumordependent resistance of rat peritoneal macrophages to herpes simplex virus Influence of the cytoskeleton on the expression of a mouse hepatitis virus (MHV-3) in peritoneal macrophages: acute and persistent infection Protection of mice against mouse hepatitis virus by Corynebacterium parvum Replication of lactic dehydrogenase virus and Sindbis virus in mouse peritoneal macrophages. Induction of interferon and phenotypic mixing Expression of herpesvirus in adherent cells derived from bone marrow of latently infected guinea pigs Bone-marrow derived macrophages as targets for the replication of mouse hepatitis virus type 3 In vitro acquisition of resistance against herpes simplex virus by permissive murine macrophages Genetics of resistance of animals to viruses: I. Introduction and studies in mice Host gene influences sensitivity to interferon action selectively for influenza virus Lethal infection with murine cytomegalovirus after early viral replication in the spleen Spontaneous activation of latent cytomegalovirus from murine spleen explants: role of lymphocytes and macrophages in release and replication of virus Pathogenesisof murine cytomegalovirus infection: the macrophage as a permissive cell for cytomegalovirus infection, replication and latency Slow virus replication: the role of macrophages in the persistence and expression of visna viruses of sheep and goats The reticuloendothelial system in scrapie pathogenesis Antibody-dependent enhancement of plaque formation on cell lines of macrophage origin-a sensitive assay for antiviral antibody 17D Yellow fever virus infection of P388D, cells mediated by monoclonal antibodies' properties of the macrophage Fc receptor Infection of a macrophagelike cell line, P388D, with reovirus; effects of immune ascitic fluids and monoclonal antibodies on neutralization and on enhancement of viral growth Role of antibodies and host cells in plaque enhancement of Murray Valley encephalitis virus Antibody-mediated enhancement of rabies virus Heterogeneity of infection enhancement of dengue 2 strains by monoclonal antibodies Infection enhancement of dengue type 2 virus in the U-937 human monocyte cell line by antibodies to flavivirus cross-reactive determinants Antibody-mediated enhancement of dengue virus infection in mouse macrophage cell lines, Mkl and Mini (41802) Complement receptor mediates enhanced flavivirus replication in macrophages Enhancement of dengue virus type 2 replication in mouse macrophage cultures by bacterial cell walls, peptidoglycans, and a polymer of pcptidoglycan subumts Delayed-type hypersensitivity: probable role in the pathogenesis of dengue hemorrhagic fever/dengue shock syndrome Dengue virus-induced cytotoxic factor induces macrophages to produce a cytotoxin MogensenSC Viral interference with the function of phagocytic cells In: FEMS Symposium of Bacterial and Viral Inhibition and Modulation of Host Defense New Acquired chemotactic inhibitors during infection with guinea pig cytomegalovirus Neutrophil response and function during acute cytomegalovirus infection in guinea pigs Synthesis of vaccinia specified antigens in mouse hepatocytes after frog virus 3-induced damage to the sinusoidal cells Effects of Sendai virus infection on function of cultured mouse alveolar macrophages Effect of dengue virus infection on Fc-receptor functions of mouse macrophages Alteration of rabbit alveolar and peritoneal macrophage function by herpes simplex virus Influenza virus replication in human alveolar macrophages Effect of infectious bovine rhinotracheitis virus infection on bovine alveolar macrophage function Depressed human monocyte function after influenza infection in vitro Acute infection of mice with lactate dehydrogenase-elevating virus enhances Fc and complement receptor activity of peritoneal macrophages Delayed wound healing in mice associated with viral alteration of macrophages Inhibition of mouse peritoneal macrophage DNA synthesis by infection with the arenavirus Pichinde Different effects of influenza virus, respiratory syncytial virus, and Sendai virus on human lymphocytes and macrophages Poliovirus-induced suppression of lymphocyte stimulation' a macrophage-mediated effect Immune stimulation, inflammation, and changes in hematopoiesis: host responses of the murine spleen to infection with cytomegalovirus Effects of antiviral agents on munne cytomegalovirus-induced macrophage dysfunction RNA synthesis in activated macrophages I. Poly(I).poly(C)-induced triggering of cytolytic activity is associated with decrease in RNA synthesis Interferon-dependent genetic resistance to influenza virus in mice: virus replication in macrophages is inhibited at an early step Interferon induces a unique protein in mouse cells bearing a gene for resistance to influenza virus Cytomegalovirus replicates in differentiated but not in undifferentiated human embryonal carcinoma cells Restriction of herpes simplex virus by macrophages. An analysis of the cell virus interaction Cloning of herpes simplex virus type 1 sequences representing the whole genome Homology between murine and human cellular DNA sequences and the terminal repetition of the S component of herpes simplex virus type 1 DNA Homology between mammalian cell DNA sequences and human herpesvirus genomes detected by a hybridization procedure with high-complexity probe Southern EM Detection of specific sequences among DNA fragments separated by gel electrophoresis Expression of feline infectious peritonitis coronavirus antigens on the surface of feline macrophagelike cells Transformation of mouse macrophages by simian virus 40 Enhanced transformation of human fibroblasts by origin-defective simian virus 40 The generation of macrophage-like cell lines by transfection with SV40 origin defective DNA The generation of human monocyte/macrophage cell lines DNA-mediated gene transfer in friend leukemia cells by cotransfection of simian virus 40 DNA with herpes simplex virus thymidine kinase DNA Macrophage nomenclature; where are we going Retroviral transforming genes in normal cells? Action of temperature-sensitive mutants of myeloproliferative sarcoma virus suggests that fibroblast-transforming and hematopoietic transforming viral properties are related Dexter TM Markedly elevated levels of an endogenous sarc protein in a hemopoietic precursor cell line Expression of cellular homologues of retroviral one genes in human hemapoietic cells Differential expression of the amv gene in human hematopoietic cells Establishment of cell lines from somatic cell hybrids between human monocytes and mouse myeloma cells Croce CM Lactic dehydrogenase virus replicates in somatic cell hybrids of mouse peritoneal macrophages and SV40-transformed human fibroblasts Virology A method for the derivation and continuous propagation of cloned murine bone marrow macrophages Replication of mouse hepatitis viruses with high and low virulence in cultured hepatocytes In vivo replication of pathogenic and attenuated strains of junin virus in different cell populations of lymphatic tissue Early replication of friend leukaemia viruses in spleen macrophages Replication of human respiratory coronavirus strain 229E in human macrophages Selective induction of enhanced complement receptor 3 activity on murine macrophages Ectoenzymes in the expression of mononuclear phagocyte differentiation mini-review Finter NB Our research in this review was partially supported by grant CA35961 from the National Cancer Institute and contract N 00014-82-K-0669 from the Office of Naval Research. We thank Ms Cindy Books for superb preparation of the manuscript and Dr Walla Dempsey for critical review.