Enhancement of disease by neutralizing antiviral antibodies in the absence of primed antiviral cytotoxic T cells

The effects of neutralizing antibodies on the disease course in mice infected with the noncytopathic lymphocytic choriomeningitis virus (LCMV) were evaluated. Whereas non-neutralizing antisera exhibiting high enzyme-linked immunosorbent assay titers had no effect on T cell responses and their consequences, neutralizing antisera modulated them variably. Neutralizing antibodies were able to prevent lethal choriomeningitis after intracerebral infection with a neurotropic LCMV-isolate (ARMSTRONG) although they could not control local virus replication. The same antibodies exhibited little or no protective effect on choriomeningitis induced by LCMV-WE, a viscerotrope isolate. Surprisingly, these antibodies rendered mice much more susceptible to choriomeningitis after intracerebral infection with LCMV DOCILE, a very rapidly spreading lymphocyto-viscerotrope virus; in this situation antibodies prevented overwhelming infection which causes deletion of immunopathogenic cytotoxic T cell responses. Thus preexisting neutralizing antiviral antibodies had little influence on local virus spread in peripheral tissues but they reduced hematogenic spread and infection of antigen-presenting cells; thereby they influenced the primary cytotoxic T cell (CTL) response and indirectly modulated the extent of T cell-mediated immunopathology in peripheral organs. These results may explain why vaccines inducing neutralizing antibodies but no CTL may enhance an immunopathological disease caused by challenge infection with a noncytopathic virus.

T cell immunity after a viral infection versus T cell tolerance induced by soluble viral peptides

The fate of in vivo activated CD8+ cytotoxic T cells was studied in transgenic mice expressing a T cell receptor (TCR) specific for the lymphocytic choriomeningitis virus (LCMV) glycoprotein peptide 33-41 presented by major histocompatibility complex (MHC) class I molecules. LCMV infection of TCR transgenic mice induced LCMV-specific effector and memory T cells whereas injection of soluble LCMV glycoprotein peptide 33-41 resulted in tolerance by peripheral deletion and anergy of LCMV-specific T cells after an initial expansion phase. Similarly, LCMV peptide 33-41-specific tolerance could be achieved in normal C57BL/6 mice and was not abrogated by an LCMV infection. These results obtained with a classically MHC-restricted peptide antigen parallel previous findings with retroviral or bacterial superantigens and indicate a possibility to modulate specifically mature peripheral cytotoxic T lymphocytes in vivo.

Lysis of infected cells in vivo by antiviral cytolytic T cells demonstrated by release of cell internal viral proteins

Immunocompetent adult mice infected with lymphocytic choriomeningitis virus (LCMV) generate a strong antiviral cytotoxic T cell response that clears virus from all organs. Although there is good evidence that immune cytotoxic T lymphocytes (CTL) kill target cells in vitro, in vivo it is debated whether antiviral activity of CD8+ T cells is mediated via direct target cell lysis or via soluble mediators. To demonstrate CD8+ T cell-mediated destruction of infected cells in vivo a specific cell-internal releasable marker was used as label, i.e. the nucleoprotein (NP) of LCMV. Since LCMV is non-cytopathic the viral NP will only be released in substantial amounts because of destruction of infected host cells by immune CTL. It is shown here that the amount of NP released from infected and 51Cr-labeled target cells in vitro correlated well with the amount of radioactivity released. Viral NP released in vivo by CTL is bound and masked by the anti-NP antibodies that are produced very early and efficiently. However, NP could readily be detected in sera of LCMV-infected CD8+ competent mice that could not generate antibodies specific for the NP because they were treated with a depleting anti-CD4 antibody. NP was also detected in the cerebrospinal fluid of mice suffering from CD8+ T cell-mediated lymphocytic choriomeningitis after intracerebral infection. NP titers in sera of anti-CD4-treated LCMV-infected mice exhibited a peak around day 7-8 when CTL activity was highest. When mice were CD8 T cell-depleted with anti-CD8 monoclonal antibody or in LCMV-carrier mice, no NP was detected in the serum. Highly activated LCMV-specific CTL adoptively transfused to LCMV-infected irradiated recipient mice also caused a time-dependent release of NP into serum. This confirms that the CD8+ population is responsible for the release of NP from infected host cells. These results represent an in vivo correlate of CTL-mediated cytolysis and evidence that antiviral cytotoxic T cells are cytolytic in vivo. They also suggest that antibody responses to internal antigens of non-cytopathic viruses may signal CD8+ T cell-mediated destruction of infected host cells.

Virus-specific cytotoxic T cell-mediated lysis of lymphocytes in vitro and in vivo

Virus-specific CTL play a major role in early antiviral protection against lymphocytic choriomeningitis virus (LCMV). When mice are infected with high doses of certain LCMV isolates, the initiated CTL response may vanish before the virus is eliminated completely. To evaluate the possibility that this may be because of CTL lysing CTL, we studied the susceptibility to lysis of LCMV-specific CTL clones and of primary immune spleen cells in vitro and in vivo. Confirming earlier reports, CTL were lysed in vitro when incubated with their specific Ag peptide; lysis was MHC restricted because T cell clones derived from allogeneic bone marrow chimeras that do not express the correct restriction element were not susceptible to mutual CTL lysis. The density of CTL cultures correlated with the degree of inactivation of CTL, indicating that CTL lysed each other mutually rather than committed suicide. To assess CTL-mediated lysis of CTL in vivo, mice were infected s.c. into the footpad with LCMV, and shortly before the CTL-dependent footpad swelling developed by day 6 to 7, specific peptide was injected locally. The regional lymph node (LN) was reduced in size, contained about 10 times fewer cells, and only 2% to 3% of the lytic activity when compared with several control LN. This drastic local effect was observed within a few hours and was accompanied by DNA fragmentation. Thus, in this model system, peptide loading of lymph node cells may lead to CTL-mediated cytotoxicity and death of T cells (including CD8+ effector cells and other LN cells) in vivo. These results may suggest that, during overwhelming virus infections, lysis of CTL passively loaded with relevant released peptides possibly may contribute to the impairment of immune responses.

Virus-specific cytotoxic T cell-mediated lysis of lymphocytes in vitro and in vivo

Virus-specific CTL play a major role in early antiviral protection against lymphocytic choriomeningitis virus (LCMV). When mice are infected with high doses of certain LCMV isolates, the initiated CTL response may vanish before the virus is eliminated completely. To evaluate the possibility that this may be because of CTL lysing CTL, we studied the susceptibility to lysis of LCMV-specific CTL clones and of primary immune spleen cells in vitro and in vivo. Confirming earlier reports, CTL were lysed in vitro when incubated with their specific Ag peptide; lysis was MHC restricted because T cell clones derived from allogeneic bone marrow chimeras that do not express the correct restriction element were not susceptible to mutual CTL lysis. The density of CTL cultures correlated with the degree of inactivation of CTL, indicating that CTL lysed each other mutually rather than committed suicide. To assess CTL-mediated lysis of CTL in vivo, mice were infected s.c. into the footpad with LCMV, and shortly before the CTL-dependent footpad swelling developed by day 6 to 7, specific peptide was injected locally. The regional lymph node (LN) was reduced in size, contained about 10 times fewer cells, and only 2% to 3% of the lytic activity when compared with several control LN. This drastic local effect was observed within a few hours and was accompanied by DNA fragmentation. Thus, in this model system, peptide loading of lymph node cells may lead to CTL-mediated cytotoxicity and death of T cells (including CD8+ effector cells and other LN cells) in vivo. These results may suggest that, during overwhelming virus infections, lysis of CTL passively loaded with relevant released peptides possibly may contribute to the impairment of immune responses.

Discrepancy between in vitro measurable and in vivo virus neutralizing cytotoxic T cell reactivities. Low T cell receptor specificity and avidity sufficient for in vitro proliferation or cytotoxicity to peptide-coated target cells but not for in vivo protection

The TCR-alpha beta of CTL recognize peptide Ag in association with MHC class I molecules. TCR binding should be highly specific to guarantee pathogen specificity and to avoid self-reactivity. Therefore, the in vivo relevance of T cells exhibiting cross-reactivities in vitro and the respective role of the TCR affinities involved are not clear. To analyze high and low avidity T cell activities both in vitro and in vivo, we investigated primary and clonal CTL responses specific for the lymphocytic choriomeningitis virus nucleoprotein 118-126 epitope in association with the two closely related H-2Ld or H-2Lq molecules. As expected, we found highly specific class I-allele-restricted CTL responses when antiviral protection or immunopathology in vivo and lysis of virus infected target cells in vitro were analyzed. In contrast, the CTL were MHC crossreactive and thus considerably less discriminatory against targets expressing high MHC-peptide densities and in proliferation assays. The data show that relatively high TCR avidities are required for virus neutralization in vivo, in contrast to in vitro analyses of peptide-coated target cells or proliferative T cell responses that may engage TCR of low avidity and broad specificity and therefore may not reflect biologically relevant TCR avidities.

Discrepancy between in vitro measurable and in vivo virus neutralizing cytotoxic T cell reactivities. Low T cell receptor specificity and avidity sufficient for in vitro proliferation or cytotoxicity to peptide-coated target cells but not for in vivo protection

The TCR-alpha beta of CTL recognize peptide Ag in association with MHC class I molecules. TCR binding should be highly specific to guarantee pathogen specificity and to avoid self-reactivity. Therefore, the in vivo relevance of T cells exhibiting cross-reactivities in vitro and the respective role of the TCR affinities involved are not clear. To analyze high and low avidity T cell activities both in vitro and in vivo, we investigated primary and clonal CTL responses specific for the lymphocytic choriomeningitis virus nucleoprotein 118-126 epitope in association with the two closely related H-2Ld or H-2Lq molecules. As expected, we found highly specific class I-allele-restricted CTL responses when antiviral protection or immunopathology in vivo and lysis of virus infected target cells in vitro were analyzed. In contrast, the CTL were MHC crossreactive and thus considerably less discriminatory against targets expressing high MHC-peptide densities and in proliferation assays. The data show that relatively high TCR avidities are required for virus neutralization in vivo, in contrast to in vitro analyses of peptide-coated target cells or proliferative T cell responses that may engage TCR of low avidity and broad specificity and therefore may not reflect biologically relevant TCR avidities.

Clonal deletion induced by either radioresistant thymic host cells or lymphohemopoietic donor cells at different stages of class I-restricted T cell ontogeny

Major histocompatibility complex (MHC) products and self-antigens expressed in the thymus determine the repertoire of mature alpha/beta T cells. While positive selection of self-MHC-restricted T cells is directed by MHC molecules expressed by thymic epithelial cells, negative selection depends to a large extent on self-antigens presented by lymphohemopoietic cells. However, radioresistant components of the thymus also influence negative selection, but it remains controversial whether this is accomplished by clonal deletion, clonal anergy, or other mechanisms. In this study, T cell development in mice expressing a transgenic T cell receptor (TCR) specific for lymphocytic choriomeningitis virus (LCMV) plus H-2Db was analyzed in the presence or absence of the viral antigen. A novel approach to analyze the thymic tissue requirements for negative selection was possible by comparing thymocyte selection in H-2Db versus H-2Dbm13 mice, since the latter allowed positive selection but not LCMV-specific deletion of transgenic TCR-expressing thymocytes. In irradiation bone marrow chimeras expressing the restriction element for negative selection (H-2Db) on host tissue, we show that radioresistant recipient cells in the thymus deleted developing T cells at an early stage of differentiation. In contrast, chimeras expressing H-2Db on lymphohemopoietic donor cells showed clonal deletion at a later stage during ontogeny.