Attrition of bystander CD8 T cells during virus-induced T-cell and interferon responses.

Experiments designed to distinguish virus-specific from non-virus-specific T cells showed that bystander T cells underwent apoptosis and substantial attrition in the wake of a strong T-cell response. Memory CD8 T cells (CD8(+) CD44(hi)) were most affected. During acute viral infection, transgenic T cells that were clearly defined as non-virus specific decreased in number and showed an increase in apoptosis. Also, use of lymphocytic choriomeningitis virus (LCMV) carrier mice, which lack LCMV-specific T cells, showed a significant decline in non-virus-specific memory CD8 T cells that correlated to an increase in apoptosis in response to the proliferation of adoptively transferred virus-specific T cells. Attrition of T cells early during infection correlated with the alpha/beta interferon (IFN-alpha/beta) peak, and the IFN inducer poly(I:C) caused apoptosis and attrition of CD8(+) CD44(hi) T cells in normal mice but not in IFN-alpha/beta receptor-deficient mice. Apoptotic attrition of bystander T cells may make room for the antigen-specific expansion of T cells during infection and may, in part, account for the loss of T-cell memory that occurs when the host undergoes subsequent infections.

Bystander sensitization to activation-induced cell death as a mechanism of virus-induced immune suppression.

Viral infections which induce strong T-cell responses are often characterized by a period of transient immunodeficiency associated with the failure of host T cells to proliferate in response to mitogens or to mount memory recall responses to other antigens. During acute infections, most of the activated, proliferating virus-specific T cells are sensitized to undergo apoptosis on strong T-cell receptor (TCR) stimulation, but it has not been known why memory T cells not specific for the virus fail to proliferate on exposure to their cognate antigen. Using a lymphocytic choriomeningitis virus (LCMV) infection model in which LCMV-immune Thy 1.1(+) splenocytes are adoptively transferred into Thy 1.2(+) LCMV carrier mice, we demonstrate here that T cells clearly defined as not specific for the virus are sensitized to undergo activation-induced cell death on TCR stimulation in vitro. This bystander sensitization was in part dependent on the expression of Fas ligand (FasL) on the activated virus-specific cells and gamma interferon (IFN-gamma) receptor expression on the bystander T cells. We propose that FasL from highly activated antiviral T cells may sensitize IFN-gamma-conditioned T cells not specific for the virus to undergo apoptosis rather than to proliferate on encountering antigen. This may in part explain the failure of memory T cells to respond to recall antigens during acute and persistent viral infections.

Minimal bystander activation of CD8 T cells during the virus-induced polyclonal T cell response.

Acute infections with viruses such as lymphocytic choriomeningitis virus (LCMV) are associated with a massive polyclonal T cell response, but the specificities of only a small percentage of these activated T cells are known. To determine if bystander stimulation of T cells not specific to the virus plays a role in this T cell response, we examined two different systems, HY-specific T cell receptor (TCR)-transgenic mice, which have a restricted TCR repertoire, and LCMV-carrier mice, which are tolerant to LCMV. LCMV infection of HY-transgenic C57BL/6 mice induced antiviral CTLs that lysed target cells coated with two of the three immunodominant epitopes previously defined for LCMV (glycoprotein 33 and nucleoprotein 397). Although LCMV-induced cytotoxic T lymphocytes (CTLs) from C57BL/6 mice could lyse uninfected H-2(k) and H-2(d) allogeneic targets, LCMV-induced CTLs from HY mice lysed only the H-2(k)-expressing cells. The HY mice generated both anti-H-2(k) and anti-H-2(d) CTL in mixed leukocyte reactions, providing evidence that the generation of allospecific CTLs during acute LCMV infection is antigen specific. During the LCMV infection there was blastogenesis of the CD8+ T cell population, but the HY-specific T cells (as determined by expression of the TCR-alpha chain) remained small in size. To examine the potential for bystander stimulation under conditions of a very strong CTL response, T cell chimeras were made between normal and HY mice. Even in the context of a normal virus-induced CTL response, no stimulation of HY-specific T cells was observed, and HY-specific cells were diluted in number by day 9 after infection. In LCMV-carrier mice in which donor and host T cells could be distinguished by Thy1 allotypic markers, adoptive transfer of LCMV-immune T cells into LCMV-carrier mice, whose T cells were tolerant to LCMV, resulted in activation and proliferation of donor CD8 cells, but little or no activation of host CD8 cells. These results support the hypothesis that the massive polyclonal CTL response to LCMV infection is virus-specific and that bystander activation of non-virus-specific T cells is not a significant component of this response.

Alpha beta and gamma delta T-cell networks and their roles in natural resistance to viral infections.

Both alpha beta and gamma delta T-cell populations and natural killer (NK) cells include cytotoxic, interferon (IFN)-gamma-producing lymphocytes that actively respond to viral infections. We show here that all three populations can provide "natural resistance" to viruses very early in infection and describe how the T-cell populations are modulated to provide this function. gamma delta T cells were shown to play a role in controlling vaccinia virus (VV) infections, as VV grew to much higher titers in gamma delta T-cell knockout mice than in normal mice 3-4 days post-infection. Our studies of the alpha beta T-cell responses to viruses revealed an interactive network of T cells that is modulated substantially during systemic infections. There is an induction phase associated with a massive virus-specific CD8 T-cell response, an apoptosis phase during which the T cells become sensitized to activation-induced cell death (AICD), a silencing phase, during which the T-cell number and activation state is reduced, and, finally, a memory phase associated with the very stable preservation of virus-specific memory cytotoxic T-lymphocyte precursors (pCTL). Infection of mice immune to one virus with a heterologous virus leads to a selective expansion of memory CTL cross-reacting between the two viruses, but, after homeostasis is again established, there is a quantitative reduction and qualitative alteration of memory to the first virus. Our results suggest that memory alpha beta T cells cross-reactive between heterologous viruses mediate both immunopathology and protective immunity at early stages of the second virus infection. Thus, memory alpha beta T cells can, like gamma delta T cells and NK cells, provide natural immunity to viral infections.

Protective CTL-dependent immunity and enhanced immunopathology in mice immunized by particle bombardment with DNA encoding an internal virion protein.

Anti-viral CTL were induced in vitro using a particle bombardment device or "gene-gun" to deliver plasmid DNA encoding the nucleoprotein of the lymphocytic choriomeningitis virus (LCMV). Using this plasmid we were able to study T cell-mediated immunity in the absence of a neutralizing Ab response. Upon a single DNA immunization, a nearly 2 log10 reduction in splenic viral titers was observed 3 days after LCMV infection. After two or three immunizations a greater than 3 log10 inhibition of viral titers in the spleen was observed, with most animals having no detectable virus. C57BL/6 mice immunized with DNA encoding the nucleoprotein gene were also challenged with LCMV intracranially. Upon intracranial challenge, vaccinated animals displayed either protection or enhanced immunopathology leading to accelerated kinetics of death. Using limiting dilution analysis it was possible to detect LCMV-specific CTL precursors in both the spleen and lymph nodes of vaccinated animals. C57BL/6 mice inoculated with DNA demonstrated an anamnestic CTL response detectable at day 4 after LCMV challenge. Thus DNA vaccines are capable of inducing an anti-viral T cell response that can inhibit viral replication and mediate either protective immunity or immunopathology. Vaccination with DNA may therefore provide a useful alternative to current viral or subunit vaccines once the efficacy of immunization with DNA is optimized.