Normal pathogen-specific immune responses mounted by CTLA-4-deficient T cells: a paradigm reconsidered.

CTLA-4 is a critical negative regulator of T cell responses and CTLA-4-deficient (CTLA-4(-/-)) mice die of a lymphproliferative disease. Nevertheless, RAG-2-deficient mice reconstituted with a mixture of CTLA-4(-/-) and normal (CTLA-4(+/+)) bone marrow survive in the absence of any signs of disease, although 50% of their T cells do not express CTLA-4. Using such mixed chimeras, we analyzed the role of CTLA-4 in specific T cell responses to lymphocytic choriomeningitis virus, Leishmania major and mouse mammary tumor virus, which cause acute, chronic and persistent infections, respectively. The populations of antigen-specific CTLA-4(-/-)CD4(+) and CTLA-4(-/-)CD8(+) T cells became activated, expanded and contracted indistinguishably from CTLA-4(+/+)CD4(+) and CTLA-4(+/+)CD8(+) T cells after infection with all three pathogens. Thus, CTLA-4 is not involved in the down-regulation of specific T cell responses and peripheral deletion in a T cell-autonomous fashion.

Cutting edge: lymphoproliferative disease in the absence of CTLA-4 is not T cell autonomous.

Mice deficient for the expression of CTLA-4 develop a lethal lymphoproliferative syndrome and multiorgan inflammation leading to death at about 4 wk of age. Here we show that RAG2-deficient mice reconstituted with CTLA-4-deficient bone marrow do not develop a lymphoproliferative syndrome despite lymphocyte infiltration mainly into pericardium and liver. Moreover, RAG2-deficient mice reconstituted with a mixture of normal and CTLA-4-deficient bone marrow remain healthy and do not develop any disease. Thus, the lethal disease observed in CTLA-4-deficient mice is not T cell autonomous and can be prevented by factors produced by normal T cells.

Influenza virus: a novel method to assess viral and neutralizing antibody titers in vitro.

The present report describes novel in vitro assays to determine influenza virus titers and virus neutralizing antibody levels. For determination of viral titers, serial dilutions of influenza virus were incubated with MDCK-cells and cultured for 48 h under a methylcellulose overlay in 24 well plates. Cells were fixed, permeabilized and stained with a monoclonal antibody specific for hemagglutitin (HA) and a peroxidase labelled second stage antibody. The sensitivity of the assay was 100-1000 times greater than a conventional hemagglutination test using fresh chicken blood. For determination of influenza virus neutralizing activity, viral samples were incubated with serial dilutions of antibody and residual viral activity was assessed in 96 well plates by the same procedure as described above. This assay made it possible to distinguish between IgM and IgG antibody titers and was about 5-10 fold more sensitive than a classical hemagglutination inhibition assay using fresh chicken blood.

OX40-deficient mice are defective in Th cell proliferation but are competent in generating B cell and CTL Responses after virus infection.

OX40, a member of the TNF receptor superfamily, is expressed on activated T cells and implicated in stimulation of T cells and T-dependent humoral responses. We generated OX40-/- mice and found that the formation of extrafollicular plasma cells, germinal centers, and antibody responses was independent of OX40. After infection with LCMV and influenza virus, OX40-/- mice retain primary and memory cytotoxic T cell responses with normal expansion and decline of specific CTL. In contrast, CD4+ T cell proliferation and the number of IFN-gamma-producing CD4+ T cells were reduced in OX40-/- mice. Moreover, the number of CD4+ T cells infiltrating the lungs of influenza virus-infected OX40-/- mice was reduced. These results define a unique role of OX40 in the generation of optimal CD4+ T cell responses in vivo.