Contributions of host and donor T cells to the inflammatory process in murine lymphocytic choriomeningitis

Lymphocytic choriomeningitis infection of the central nervous system

Viral infection of the central nervous system (CNS) can result in a multitude of responses including pathology, persistence or immune clearance. Lymphocytic choriomeningitis virus (LCMV) is a powerful model system to explore these potential outcomes of CNS infection due to the diversity of responses that can be achieved after viral inoculation. Several factors including tropism, timing, dose and variant of LCMV in combination with the development or suppression of the corresponding immune response dictates whether lethal meningitis, chronic infection or clearance of LCMV in the CNS will occur. Importantly, the functionality and positioning of the LCMV-specific CD8+ T cell response are critical in directing the subsequent outcome of CNS LCMV infection. Although a basic understanding of LCMV and immune interactions in the brain exists, the molecular machinery that shapes the balance between pathogenesis and clearance in the LCMV-infected CNS remains to be elucidated. This review covers the various outcomes of LCMV infection in the CNS and what is currently known about the impact of the virus itself versus the immune response in the development of disease or clearance.

Antigen-specific CD8+ T cells mediate a peptide-induced fatal syndrome

Peptide immunotherapy both activates and suppresses the T cell response against known peptide Ags. Although pretreatment with VP2(121-130) peptide inhibits the development of antiviral CTL specific for the immunodominant D(b):VP2(121-130) epitope expressed during acute Theiler's murine encephalomyelitis virus infection, i.v. injection of this same peptide or MHC tetramers containing the peptide during an ongoing antiviral CTL response results in a peptide-induced fatal syndrome (PIFS) within 48 h. Susceptibility to PIFS is dependent on peptide-specific CD8(+) T cells, varies among inbred strains of mice, and is not mediated by traditionally defined mechanisms of shock. Analyses using bone marrow chimeras and mutant mice demonstrate that susceptibility to PIFS is determined by the genotype of bone marrow-derived cells and requires the expression of perforin. Animals responding to peptide treatment with PIFS develop classical stress responses in the brain. These findings raise important considerations for the development of peptide therapies for active diseases to modify immune responses involving expanded populations of T cells. In summary, treatment with peptides or MHC-tetramers during a peptide-specific immune response can result in a fatal shock-like syndrome. Susceptibility to the syndrome is genetically determined, is mediated by CD8(+) T cells, and requires expression of perforin. These findings raise concerns about the use of peptides and MHC tetramers in therapeutic schemes.

T cells in the central nervous system: the delicate balance between viral clearance and disease

The central nervous system (CNS) is considered an "immunoprivileged" site with restricted access and a unique microenvironment that profoundly affects the capacity of T cells to exert their functions. The lymphocytic choriomeningitis virus model offers a unique system in which to evaluate the contrasting roles of specific T cells in causing lethal CNS disease or curing pervasive and life-long CNS infection. Specific T cell kinetics in the periphery is briefly discussed. The T cell-mediated mechanisms leading to fatal choriomeningitis are reviewed as are recent methodologic advances that will facilitate the study of antigen-specific T cells in disease pathogenesis. Understanding the specific constraints imposed by the CNS on local T cell activity has important consequences for the design of therapeutic strategies aimed at preventing or curing CNS infection.

Regulation of T cell responses during central nervous system viral infection

This chapter focuses on the contribution of T cells to the pathogenesis of neurologic disease and discusses specific examples of how individual T cell effector functions can be regulated during central nervous system's (CNS) viral infections. T cells can serve a variety of functions as part of the host immune response during CNS viral infection. They can participate directly in viral clearance from the brain, or they can promote the survival of the host without exerting any direct effect on virus replication. Only a small number of T cells infiltrate the brain under normal circumstances. This paucity of immune surveillance of baseline is one of several reasons why the CNS has often been characterized as an “immunologically privileged” site. T cell-mediated lysis of infected cells has been demonstrated to be an important mechanism of viral clearance from tissues other than the CNS. In several well-characterized animal models of CNS viral infection, part of the elicited T cell response actually contributes to the pathology and adverse outcome of disease. Neurotropic lymphocytic choriomeningitis virus infection of adult mice is the premier example of this phenomenon.

The role of antigen in the localization of naive, acutely activated, and memory CD8(+) T cells to the lung during influenza pneumonia

The role of Ag in the recruitment and localization of naive, acutely activated, and memory CD8(+) T cells to the lung during influenza infection was explored using TCR-transgenic (Tg) mice. Naive, Thy1.2(+)CD8(+) OT-I TCR-Tg cells were primed and recruited to the lung after transfer into congenic Thy1.1(+) recipients challenged with a genetically engineered influenza virus (influenza A/WSN/33 (WSN)-OVA(I)) containing the K(b) restricted OVA(257-264) epitope (siinfekl) in the viral neuraminidase stalk. However, if the transferred animals were infected with a similar influenza virus that expressed an irrelevant K(b) epitope (WSN-PEPII), no TCR-Tg T cells were detectable in the lung, although they were easily visible in the lymphoid organs. Conversely, there were substantial numbers of OT-I cells found in the lungs of WSN-PEPII-infected mice when the animals had been previously, or were concurrently, infected with a recombinant vaccinia virus expressing OVA. Similar results were obtained with nontransgenic populations of memory CD8(+) T cells reactive to a murine gamma-herpesvirus-68 Ag. Interestingly, the primary host response to the immunodominant influenza nucleoprotein epitope was not affected by the presence of memory or recently activated OT-I T cells. Thus, although Ag is required to activate the T cells, the subsequent localization of T cells to the lung during a virus infection is a property of recently activated and memory T cells and is not necessarily driven by Ag in the lung.

Preservation of motor function by inhibition of CD8+ virus peptide-specific T cells in Theiler's virus infection

Central nervous system-infiltrating CD8+ T cells are potential mediators of neuropathology in models of multiple sclerosis induced by Theiler's murine encephalomyelitis virus (TMEV) infection. C57BL/6 mice mount a vigorous cytotoxic T lymphocyte (CTL) response against the immunodominant virus peptide VP2121-130 and clear TMEV infection. Interferon-g (IFN-g)R-/- mice also mount a strong CTL response against the VP2121-130 epitope, but because of genetic deficiencies in critical IFN-g signaling pathways, they do not clear TMEV infection and develop prominent neurological deficits within 6 wk. This pronounced disease process, coupled with a defined CTL response, provides an ideal model for evaluating the importance of antiviral CTL activity in the development of severe demyelination and loss of motor neuron function. By administering the VP2121-130 peptide before and during TMEV infection, 99% of the VP2121-130-specific CD8+ T cell response was inhibited. No decrease in virus infection was observed. Peptide treatment did result in significantly less motor dysfunction, even when no differences in levels of demyelination were observed. Although most investigators focus on the role of CD4+ T cells in demyelinating disease, these studies are the first to demonstrate a clear contribution of antiviral CD8+ T cells in neurological injury in a chronic-progressive model of multiple sclerosis.

Effector CD4+ and CD8+ T-cell mechanisms in the control of respiratory virus infections

The rules for T-cell-mediated control of viruses that infect via the respiratory mucosae show both common themes and differences, depending on the nature of the pathogen. Virus-specific CD8+ cytotoxic T lymphocytes (CTLs) are the key effectors of virus clearance in mice infected with both negative strand RNA viruses (influenza and Sendai) and a DNA virus, the murine gamma-herpesvirus-68 (MHV-68). Recently completed experiments establish that these activated CD8+ T cells indeed operate primarily via contact-dependent lysis. Perforin-mediated cytotoxicity seems to be the preferred mode, though a Fas-based mechanism can apparently serve as an alternative mechanism. Immune CD4+ T cells functioning in the absence of the CD8+ subset cannot eliminate MHV-68 from lung epithelial cells, are somewhat less efficient than the CD8+ CTLs at clearing the RNA viruses, and are generally ineffectual in mice that lack B lymphocytes. Though cytokine secretion by CD4+ and CD8+ T cells in the virus-infected lung may promote both T-cell extravasation and macrophage activation, such processes are not alone sufficient to deal consistently with any of these infections. However, CD4+ T help is mandatory for an effective B-cell response, and can operate to promote the clonal expansion of virus-specific CD8+ T cells in the lymph nodes and spleen. Furthermore, a concurrent CD4+ T-cell response seems to be essential for maintaining continued CD8+ T-cell surveillance and effector capacity through the persistent, latent phase of MHV-68 infection in B cells. Thus, the evidence to date supports a very traditional view; CD8+ T cells function mainly as killers and the CD4+ T cells as helpers in these respiratory virus infections.

Molecular mechanisms of lymphocyte-mediated cytotoxicity and their role in immunological protection and pathogenesis in vivo

Studies with perforin-deficient mice have demonstrated that two independent mechanisms account for T cell-mediated cytotoxicity: A main pathway is mediated by the secretion of the pore-forming protein perforin by the cytotoxic T cell, whereas an alternative nonsecretory pathway relies on the interaction of the Fas ligand that is upregulated during T cell activation with the apoptosis-inducing Fas molecule on the target cell. NK cells use the former pathway exclusively. The protective role of the perforin-dependent pathway has been shown for infection with the noncytopathic lymphocytic choriomeningitis virus, for infection with Listeria monocytogenes, and for the elimination of tumor cells by T cells and NK cells. In contrast, perforin-dependent cytotoxicity is not involved in protection against the cytopathic vaccinia virus and vesicular stomatitis virus. LCMV-induced immunopathology and autoimmune diabetes have been found to require perforin-expression. A contribution of perforin-dependent cytotoxicity to the rejection of MHC class I-disparate heart grafts has also been observed. Its absence is efficiently compensated in rejection of fully allogeneic organ or skin grafts. So far, evidence for a role of Fas-dependent cytotoxicity as a T cell effector mechanism in vivo is lacking. Current data suggest that the main function of Fas may be in regulation of the immune response and apparently less at the level of an effector mechanism in host defense. Further analysis is necessary, however, to settle this point finally.

Limiting the available T cell receptor repertoire modifies acute lymphocytic choriomeningitis virus-induced immunopathology

The invariably fatal immunopathological disease that follows intracerebral injection of CBA/Ca (H-2k) mice with 1000 PFU of lymphocytic choriomeningitis virus (LCMV) generally fails to develop in congenic mice transgenic for a V beta 8.1D beta 2J beta 2.3C beta 2 T cell receptor (TCR) gene. The majority of these LCMV-infected TCR-transgenic mice show a substantial meningitis of delayed onset, that resolves without causing any obvious clinical impairment. This inflammatory process depends on the involvement of V beta 8+ T cells, but does not require the participation of the CD4+ subset. The cytotoxic effectors that develop in both the transgenic mice and the CBA/Ca controls are lytic for target cells infected with a vaccinia construct expressing genes encoding the putative polymerase protein of LCMV. Limiting the available TCR repertoire to lymphocytes with a single V beta phenotype (not required for the generation of potent effectors in wild-type mice) thus modifies the development of the lethal neuropathology characteristic of LCMV infection, although the CD8+ cytotoxic T lymphocyte response is not greatly compromised.

Lymphocytic choriomeningitis virus induces a chronic wasting disease in mice lacking class I major histocompatibility complex glycoproteins

Lymphocytic choriomeningitis virus (LCMV) induces a chronic, wasting syndrome when injected intracerebrally into H-2b mice homozygous for a beta 2-microglobulin (beta 2-m (-/-)) gene disruption. These mice have very few CD8+ T cells and express little class I MHC glycoprotein, though minimal levels of the H-2Db molecule have been detected on in vitro cultured beta 2-m (-/-) cells. The underlying immunopathological process in these beta 2-m (-/-) mice is mediated by virus immune CD4+ effectors. However, adoptively transferred CD8+ T cells from normal, LCMV-infected H-2Db compatible donors induce significant (but low level) meningitis in beta 2-m (-/-) recipients. Such mice develop neither the neurological disease characteristic of LCM nor the persistent, though generally non-fatal, debility that occurs when only the CD4+ T cell subset is involved.

Antiviral immune responses of lymphocytic choriomeningitis virus-infected mice lacking CD8+ T lymphocytes because of disruption of the beta 2-microglobulin gene

Mice infected intracerebrally with lymphocytic choriomeningitis virus (LCM virus) develop a characteristic central nervous system disease and usually die. If the intravenous or intraperitoneal route is used, the infection leads to less severe clinical signs and the virus is eliminated. Illness and virus clearance are immunological phenomena, which are assumed to be caused exclusively by CD8+ T lymphocytes. In contrast, of the two phases of a delayed-type hypersensitivity reaction caused by inoculation of the virus into the mouse's foot, only the first is mediated by CD8+ cells, whereas the second is mediated by CD4+ cells. We have examined LCM virus-specific immune responses in mice devoid of CD8+ T lymphocytes as a result of disruption of the beta 2-microglobulin gene. As expected, the virus persisted but footpad swelling did not occur, although intracerebral infection resulted in CD4+ T-lymphocyte-mediated illness and antiviral antibodies were produced. Different results had been obtained by Fung-Leung et al. (W.-P. Fung-Leung, T. M. Kündig, R. M. Zinkernagel, and T. W. Mak, J. Exp. Med. 174:1425-1429, 1991), who, is essentially identical experiments but with mice lacking CD8+ T lymphocytes as a result of disruption of the Lyt-2-encoding gene, recorded control of the infection and development of a local delayed-type hypersensitivity reaction. We consider these differences important, because they provide us with clues that may help to understand the mode of action of the CD8+ T cells in cell-mediated antiviral immunity.

Dissection of an inflammatory process induced by CD8+ T cells

A massive delayed type hypersensitivity (DTH) reaction occurs in the cerebrospinal fluid (CSF) of mice with lymphocytic choriomeningitis (LCM). In this article, Peter Doherty and colleagues analyze this reaction together with the population dynamics of the regional lymph node to give a comprehensive picture of the events underlying this CD8+ T-cell-mediated immunopathological disease. Their findings are of general relevance to the understanding of inflammation.

Influence of non-major histocompatibility complex differences on the severity of lymphocytic choriomeningitis

The role of the non-major histocompatibility complex (MHC) genetic background in the development of lymphocytic choriomeningitis (LCM) was examined for a range of mouse strains of the H-2k haplotype. The onset of meningitis relative to the time of injection of LCM virus was delayed and the maximal level of cellular extravasation into cerebrospinal fluid was lower in C3H/HeJ and CBA/H compared with AKR/J, B10.Br and BALB/c.H-2k mice. Adoptive transfer experiments indicated that the C3H mice are genuine low responders, but immune spleen cells from the CBA/H were as potent on a cell-for-cell basis as those from the AKR/J. Further analysis with CBA/H, AKR/J and (CBA/H x AKR/J)F1 mice showed that the pattern of high response for the AKR/J was dominant, with the differential kinetics of the development of meningitis correlating with the cellularity of the cervical lymph nodes. Thus, the generation of the LCM inflammatory process is not dictated solely by the MHC phenotype.

Consequences of exposure to ionizing radiation for effector T cell function in vivo

The adoptive transfer of actuely primed and memory virus-immune CD8+ T cells causes enhanced meningitis in both cyclophosphamide (Cy) suppressed, and unsuppressed, recipients infected with lymphocytic choriomeningitis virus (LCMV). The severity of meningitis is assessed by counting cells in cerebrospinal fluid (CSF) obtained from the cisterna magna, which allows measurement of significant inflammatory process ranging from 3 to more than 300 times the background number of cells found in mice injected with virus alone. Exposure of the donor immune population to ionizing radiation prior to transfer has shown that activated T cells from mice primed 7 or 8 days previously with virus may still promote a low level of meningitis in unsuppressed recipients following as much as 800 rads, while this effect is lost totally in Cy-suppressed mice at 600 rads. Memory T cells are more susceptible and show no evidence of in vivo effector function in either recipient population subsequent to 400 rads, a dose level which also greatly reduces the efficacy of acutely-primed T cells. The results are interpreted as indicating that heavily irradiated cells that are already fully functional show evidence of primary localization to the CNS and a limited capacity to cause pathology. Secondary localization, and events that require further proliferation of the T cells in vivo, are greatly inhibited by irradiation.