Investigation of antigen-specific CD4+ T cell immune repertoire and the relationship between TCR affinity and effector function

© 2014 Dr. Darryl Neil JohnsonT cells recognise peptide antigens displayed in complex with MHC molecules via a T cell receptor (TCR). The antigen-specific responses of both CD8+ and CD4+ T cells have been investigated previously, however the nature of the development of antigen-specific CD4+ T cell responses during infection is not clearly understood and requires further study. To this end, a mouse model of cutaneous HSV-1 infection was utilised and the development of the CD4+ T cell response specific to residues 315-327 of HSV-1 glycoprotein D (gD315-327) was assessed. Antigen-specific T cell populations can consist of cell expressing clonally distinct TCRs. The gD315-327-specific CD4+ T cell repertoire consisted of a very broad range of Vβ gene segments and a highly diverse range of CDR3β motifs. Furthermore, little similarity was observed either between the repertoires collected within the priming LN and peripheral sites or between the repertoires of individual mice. Repertoire diversity to the degree found within the gD315-327-specific CD4+ T cell response has not been observed previously within any other specific CD4+ T cell repertoires. From the available biochemical measurements of CD4+ T cell TCRs it appears that MHC-II restricted TCRs can have broad range of affinities. Furthermore, it is possible that specific CD4+ T cell responses with high diversity have the greatest potential to contain TCRs with such a broad range of affinities. Indeed, when the affinities of two gD315-326-specific TCRs, which had previously been identified following CD4+ T cell responses during the cutaneous mouse model, were measured it was found that each recognised gD315-327:I-Ab with very different affinities. Not only is TCR/pMHC engagement critical for T cell activation, but the strength with which TCRs bind pMHC has also been observed to affect T cell expansion and the development of effector functions. High affinity TCR/pMHC-II interactions have been associated with greater antigen-specific CD4+ T cell expansion and preferential development of TH subsets such as TFH and TH1 subsets. In this thesis the relationship between TCR affinity and antigen-specific CD4+ T cell function following infection was investigated within the mouse model by utilising two gD315-327-specific CD4+ T cell clones and altered gD315-327 peptides. The two antigen specific clones expressed TCRs that differ greatly in affinity, while the altered peptides resulted in the reduced functional avidity of the CD4+ T cell clones. Together, the in vivo responses of the gD315-327-specific CD4+ T cells following HSV-1 infection or altered peptides, delivered with engineered influenza viruses, indicated that TCR affinity does influence CD4+ T cell expansion and function. Lower affinity TCR/pMHC-II interactions were associated with a reduction in the magnitude of the specific CD4+ T cell populations. Furthermore, the frequency of the gD315-327-specific CD4+ T cells with the PD-1HiCXCR5Hi TFH phenotype or expressed T-bet were reduced with lower affinity. The studies here provide insights into the repertoire diversity of an antigen-specific CD4+ T cell response and also into the nature of the relationship between TCR affinity and CD4+ T cell effector functions