T-cell intrinsic expression of MyD88 is required for sustained expansion of the virus-specific CD8+ T-cell population in LCMV-infected mice.

Acute infection with lymphocytic choriomeningitis virus (LCMV) normally results in robust clonal expansion of virus-specific CD8(+) T cells, which in turn control the primary infection. However, similar infection of myeloid differentiation factor 88 (MyD88)-deficient mice leads to a markedly impaired T-cell response and chronic infection. It has been found previously that impairment of the innate immune response is not sufficient to explain this profound change in outcome. Using adoptive transfer of CD8(+) T cells, this study demonstrated unequivocally that T-cell expression of MyD88 is critical for a normal T-cell response to LCMV. In addition, it was found that expression of MyD88 is superfluous during early activation and proliferation of the antigen-activated CD8(+) T cells, but plays a critical role in the sustained expansion of the antigen-specific CD8(+) T-cell population during the primary T-cell response. Interestingly, a critical role for MyD88 was evident only under conditions of systemic infection with virus capable of causing prolonged infection, suggesting that MyD88 expression may function as an internal regulator of the threshold for antigen-driven, exhaustive differentiation.

Fulminant lymphocytic choriomeningitis virus-induced inflammation of the CNS involves a cytokine-chemokine-cytokine-chemokine cascade.

Intracerebral inoculation of immunocompetent mice with lymphocytic choriomeningitis virus (LCMV) normally results in fatal CD8+ T cell mediated meningoencephalitis. However, in CXCL10-deficient mice, the virus-induced CD8+ T cell accumulation in the neural parenchyma is impaired, and only 30-50% of the mice succumb to the infection. Similar results are obtained in mice deficient in the matching chemokine receptor, CXCR3. Together, these findings point to a key role for CXCL10 in regulating the severity of the LCMV-induced inflammatory process. For this reason, we now address the mechanisms regulating the expression of CXCL10 in the CNS of LCMV-infected mice. Using mice deficient in type I IFN receptor, type II IFN receptor, or type II IFN, as well as bone marrow chimeras expressing CXCL10 only in resident cells or only in bone marrow-derived cells, we analyzed the up-stream regulation as well as the cellular source of CXCL10. We found that expression of CXCL10 initially depends on signaling through the type I IFN receptor, while late expression and up-regulation requires type II IFN produced by the recruited CD8+ T cells. Throughout the infection, the producers of CXCL10 are exclusively resident cells of the CNS, and astrocytes are the dominant expressors in the neural parenchyma, not microglial cells or recruited bone marrow-derived cell types. These results are consistent with a model suggesting a bidirectional interplay between resident cells of the CNS and the recruited virus-specific T cells with astrocytes as active participants in the local antiviral host response.

Perforin and IFN-gamma do not significantly regulate the virus-specific CD8+ T cell response in the absence of antiviral effector activity.

Using gene-targeted mice we have investigated whether perforin and/or interferon-gamma exert a direct regulatory effect on the expansion and contraction of antigen-specific CD8(+) T cells following infection with a virus (vesicular stomatitis virus) which is not controlled through these molecular effector systems. Unlike what has been observed when these molecules are essential for pathogen clearance, neither molecule was found to play an important role in regulating the kinetics of the virus-specific CD8(+) T cell response in the absence of antiviral effector activity.

Role of CD28 co-stimulation in generation and maintenance of virus-specific T cells.

Efficient induction of T cell responses is normally assumed to require both TCR-mediated signaling and engagement of co-stimulatory molecules, in particular CD28. However, the importance of CD28 co-stimulation in induction and maintenance of antiviral T cell responses is not clearly established. For this reason antiviral CD4(+) and CD8(+) T cell responses in CD28-deficient mice were studied using two different viruses [vesicular stomatitis virus and lymphocytic choriomeningitis virus (LCMV)]. Intracellular cytokine staining and/or MHC-peptide tetramers were used to enumerate antigen-specific T cells. In addition, we used DNA constructs encoding viral epitopes to probe the importance of the epitope itself. Our results reveal that while the context of antigen presentation (live virus versus DNA construct) is a critical factor in determining the requirement for CD28 co-stimulation, epitope and virus dose play little if any role. Direct visualization of antigen-specific cells also confirms the notion that CD28 is more critical for the generation of antiviral T(h)1 cells than for T(c)1 cells generated in response to the same virus (LCMV). Most importantly, the present study reveals that CD28 generally is essential for the host to respond optimally over a broad set of conditions, and our results may imply that the relatively CD28 independent activation of LCMV-specific CD8(+) T cells may represent an extreme situation related to the non-cytolytic nature of this virus allowing the delivery of a uniquely strong and prolonged signal 1.