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Lymphocytic choriomeningitis (LCM) virus of the mice provides perhaps the most extensively studied model of viral-induced immunopathology. Intraocular inoculation of this virus in the adult mouse produces a severe and progressive uveitis with involvement also of the cornea and retina. Intracerebral infection of the newborn rat with LCM virus produces a severe retinopathy, with only minimal involvement of the uveal tract. We review in this paper recent studies on these two animal models of LCM virus disease which demonstrate: (1) the histopathology of the ocular lesion; (2) that immunosuppression after infection prevents development of the disease; (3) that passive transfer of sensitized lymphoid cells to the immunosuppressed infected animal can then trigger ocular disease, and (4) that T lymphocytes, and not circulating antibody, play the major role in the pathogenesis of this disease process.

Cloned lymphocytic choriomeningitis virus (LCMV)-specific cytotoxic T lymphocyte (CTL) lines were prepared from spleens of 129/J (H-2b) mice immunized 7-9 months earlier with LCMV (UBC strain), or of C57BL/10J (H-2b) mice immunized 4 to 5 weeks earlier with LCMV (Armstrong strain). One uncloned and 3 cloned cytotoxic T cell lines were assessed for their respective abilities to produce, or protect against, fatal disease upon transfer to appropriate recipients or to induce specific footpad-swelling reaction. The effects of all lines were essentially identical. In recipient mice acutely infected with LCMV and immunosuppressed either by irradiation (750-990 rds) or treatment with cyclophosphamide, cloned T cells administered intracerebrally (i.c.) caused a convulsive disease and death within 1-4 days. No disease was produced when the same CTL were transferred to uninfected recipients or when they had been frozen and thawed prior to transfer to infected recipients. When admixed with 500 plaque-forming units of LCMV and transferred i.c. to immunocompetent H-2b mice, the T cell clones prevented overt disease. Allogeneic (H-2k) recipients of this same admixture all developed typical LCM disease as did H-2b recipients of the admixture after T cells had been frozen and thawed. Inoculation of cloned CTL into preinfected footpads induced a specific footpad-swelling reaction, which reached maximum levels after about 36 h. Irradiated and infected recipients of cloned LCMV-specific T cells showed the footpad-swelling reaction only when they had been reconstituted with bone marrow cells. In contrast, cloned T cells induced LCM disease in i.c. infected and irradiated mice independent of bone marrow reconstitution. These findings indicate that both fatal LCMV-induced neurologic disease and protection against it are mediated directly by virus-specific CTL.

Intracerebral inoculation of 4-day-old rats with lymphocytic choriomeningitis virus (E-350 strain) produced a nonfatal, acute, severe, and permanent cerebellar necrosis with minimal histological evidence of inflammation. Virus persisted in the brain at high titers for 30 to 40 days and was finally cleared about 120 days after infection. Rabbit anti-rat lymphoid serum prevented cerebellar necrosis, and brain virus titers were higher than in control animals receiving normal rabbit serum. Thus, the immune response which clearly is responsible for the cerebellar lesion also plays a role in the gradual suppression of infection.

Mice with self-limiting P. yoelii or fatal P. berghei infections exhibited a markedly impaired ability to mount specific splenic cytotoxic T-lymphocyte responses to immunization with infectious ectromelia (EV), vaccinia (VAC), or lymphocytic choriomeningitis viruses (LCMV). Lymph node responsiveness, however, was not impaired. Primary CTL responses were depressed in mice immunized 7 days after P. berghei infection, while in P. yoelii-infected mice, depressed responses were detected only during the period corresponding with maximal parasitemia (days 9-12). Secondary VAC-specific CTL responses in vitro by spleen cells of mice previously immunized during P. yoelii infection were also depressed if UV-inactivated rather than infectious VAC was used for immunization. In addition, spleen cells of mice already immune to VAC failed to yield normal secondary CTL responses in vitro during the period of maximal P. yoelii parasitaemia. Collectively, these findings indicate that, during patent malaria infections, priming for and expression of virus-specific CTL responses may be inhibited.

Sublethal irradiation (500 R) of C3H mice is followed by a gradual replacement of radiosensitive cells in their spleens by surviving stem cells originating in bone marrow. This compensatory hemopoiesis was quantitated by counting the numbers of stem cell-derived colonies appearing on spleen surfaces, as well as those which grew in vitro after marrow cells, suspended in soft agar, were overlaid onto syngenic mouse embryo fibroblast feeder layers. Compensatory colony formation, both in vivo and in vitro, was severely depressed when mice were infected with lymphocytic choriomeningitis virus (LCMV) 1 day before irradiation, although the induction of virus-specific cytotoxic T cells in their spleens was unimpaired. Without irradiation, mice, acutely infected with LCMV, showed a dramatic reduction in the numbers of specific antibody-forming cells generated in their spleens after priming with sheep erythrocytes during week 1 post-infection, yet the ability of their marrow cells to form colonies in vitro remained normal. Therefore, the basis of immunodepression is distinct from that of defective hemopoiesis since the latter is apparent only when LCMV infection is accompanied by irradiation. However, as discussed, both phenomena may be related to alterations induced within the splenic environment by LCMV.

Primary immunizing infections with LCM virus result in a transient depression of the in vitro proliferative responses of splenic lymphocytes to mitogens specifically reactive with T cells or B cells. This depression of lymphocyte function is the result of a virus-induced defect in an adherent, phagocytic cell population required for in vitro lymphocyte activation. Depressed responses persist for about 1 week after virus clearance and can be corrected by the addition of normal PEM or 2-ME to infected spleen cell cultures. Although the precise nature of this defect remains unclear, it is postulated that it is due to a productive infection of macrophages and their precursors that renders them dysfunctional. Secondary LCM virus infections do not result in depressed in vitro responses to mitogens, presumably because of rapid virus clearance and limited numbers of infected cells. Primary infections of immunologically immature mice, mice rendered functionally athymic, or mice treated with nonspecific immunosuppressive agents result in LCM virus persistence. Shortly after infection, these animals show a similar depression of immunologic reactivity that returns to normal as the virus carrier state becomes established. Despite virus persistence, few PEM from established LCM virus carrier mice contain viral antigens and these cells function normally. Thus, LCM virus-induced immunosuppression appears to reflect a subtle cytopathic effect of LCM virus replication that is not mediated by the virus-specific cell-mediated immune mechanisms responsible for acute LCM virus disease.

At varying intervals after immunizing infections of adult BALB/c mice with lymphocytic choriomeningitis (LCM) virus, splenic lymphocytes were tested for their ability to either elicit acute LCM or protect against lethal intracerebral LCM virus challenge when transferred to syngeneic recipients that were, respectively, virus carriers induced by cyclophosphamide-induced immunosuppression and normal susceptible mice. These lymphocytes were also assayed for their capacity to lyse, in vitro, LCM virus-infected BALB 3T3 cells labeled with 51Cr. Only lymphocytes obtained from donor mice between 6 and 8 days postimmunization significantly protected normal recipients from the development of lethal central nervous system (CNS) disease when transferred 18-24 h before virus challenge. At 10 days they were not protective even though their cytolytic activity in vitro either exceeded or approximated that of protective lymphocytes. The capacity to protect more closely corresponded with a period of virus-induced DNA synthesis in donor spleens as measured by the incorporation of [5--125I]-2'-DEOXYURIDINe. However, none of these cytolytically active lymphocyte populations were effective in mediating acute CNS disease when transferred to virus-carrier mice. In contrast, lymphocytes obtained 18 days or later after primary immunization, although having no protective capacity and exhibiting minimal cytolytic activity in vitro, were able to regularly produce acute disease in virus-carrier mice. The ablation by anti-theta-serum treatment of these in vivo and in vitro virus-specific immune functions established that the relevant lymphoid cells were T lymphocytes. Similarly, reconstitution of C3H times C57BL F1 adult mice, depleted of lymphocytes by thymectomy and lethal irradiation, with syngeneic day 8 immune donor lymphocytes rendered them resistant to the development of the carrier state after intracerebral virus challenge; reconstitution of such animals with nonimmune lymphocytes restored their ability to develop typical LCM. Collectively, these data indicate that immunogenic stimulation with LCM virus leads to the development of at least two different virus-specific T-lymphocyte subsets: (a) an early appearing transient effector population of cells and, perhaps, their immediate precursors whose generation requires the presence of virus, and (b) a stable memory population that appears well after virus clearance and which has little or not no cytolytic activity. The possible mechanism by which these functionally different subsets can influence the outcome of acute or chronic LCM virus infections is discussed.