T cells Masquerading as Antigen-Presenting Cells

Activation of T cells is normally promoted by interaction of T cell receptors with complexes of antigenic peptide bound to MHC (major histocompatibility complex) proteins on the surface of antigen-presenting cells (APCs). However, Ge et al. report that activation of naïve or CD8 + T cells can occur with exposure to soluble peptide-MHC complexes in the absence of APCs. Their results indicated that activation actually occurred through transfer of peptides to MHC molecules expressed by the T cells. Thus, T cells were only activated if they expressed the same MHC as the one with which they were stimulated. Furthermore T cells that had been exposed to MHC-peptide monomers and then washed could subsequently activate naïve T cells, apparently by presenting the peptide antigen on their cell surface as would a professional APC. Given that soluble peptide-MHC complexes are likely to exist in serum or synovial fluid, the authors propose that such presentation of antigen by T cells in vivo could influence T cell maturation. Q. Ge, J. D. Stone, M. T. Thompson, J. R. Cochran, M. Rushe, H. N. Eisen, J. Chen, L. J. Stern, Soluble peptide-MHC monomers cause activation of CD8 + T cells through transfer of the peptide to T cell MHC molecules. Proc. Natl. Acad. Sci. U.S.A. 99 , 13729-13734 (2002). [Abstract] [Full Text]