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.     

Role of the H-2I region in the generation of an antiviral cytotoxic T-cell response in vitro

The participation of H-2I gene products in generating virus-specific proliferative and/or cytotoxic T-lymphocyte (CTL) responses was investigated. Spleen cells from mice infected with vaccinia virus were restimulated secondarily in vitro with syngeneic virus-infected peritoneal exudate cells (PEC) and then restimulated in tertiary cultures with virus-infected PEC from syngeneic and partially histoincompatible strains of mice. Based on the finding that comparable proliferative responses resulted when stimulating the responding cells were histocompatible at the H-2K, I, or D region of the major histocompatibility complex (MHC), the additively enhanced, but not potentiated, proliferation caused by identity at two or three H-2 regions was analyzed. Enhancement of proliferation followed when the H-2K/D components plus virus and the H-2I components plus virus were either on the same, or alternatively on two, stimulating cells. This suggests that H-2K, D, and I plus virus trigger distinct T-cell subsets. A virus-specific CTL response was generated in vitro when spleen cells from virus-primed mice and even unprimed mice were stimulated with cells sharing only H-2K and/or H-2D of the MHC. Identity at the H-2I region did not stimulate a CTL response, nor did it influence the magnitude of the $\text{K}\text{D}$ restricted response. Nevertheless, the presence of anti-Ia antiserum in cultures of syngeneic stimulators and responders inhibited the cytotoxic response to a great extent. Therefore, H-2I region products seem to participate in the generation of virus-specific CTL in vitro.     

Virus-specific proliferative T-cell responses: Parameters and specificity

The genetic requirements for inducing virus-specific T-cell proliferation were investigated by taking spleen cells from animals primed with vaccinia virus in vivo, then culturing the cells in vitro with vaccinia virus-infected syngeneic peritoneal macrophages, and finally restimulating these cells a second time in vitro with vaccinia virus-infected macrophages from several strains of mice. Under these conditions, T cells proliferated in the tertiary response to virus-specific stimulation, whereas background proliferation caused by allogeneic differences between stimulator and responder cells was minimal. Compatibility between T cells and infected stimulator cells at the K or I regions alone or at I-A or I-A + I-B regions of the major histocompatibility complex (MHC) produced strong proliferative responses, whereas compatibility at D alone often resulted in somewhat weaker responses. However, these responses were rarely as great as in combinations of completely syngeneic stimulator and responder cells. Homology between responding and virus-infected stimulating cells in more than one of the H-2K, D, or I regions resulted in an additive, but not potentiating, effect. Genes coded outside the H-2 region did not seem to play a role in this system. In some rare cases, a weak response occurred across allogeneic barriers, but in general, virus-specific T-cell proliferation was strongly H-2 restricted.