Localization of soluble major histocompatibility class II-peptide complexes on T cell surface

Affinity purified major histocompatibility (MHC)-peptide complexes are heterodimeric cell surface glycoproteins and are known to recognize antigen-specific CD4(+) T cell receptors (TCRs). In general, the affinity of MHC-peptide complexes to TCRs are considered very low with a K(D) of 5 x 10(-5) M and, therefore, stabilization of these complexes on T cell surface was not reported earlier. This could be due to (1) incomplete occupancy of MHC molecules with antigenic peptides, (2) variability of the binding constant of peptides to MHC molecules, (3) presence of endogenously bound peptides in MHC preparations, or (4) a combination of these. Using well-characterized HLA-DR2 complex loaded with a high affinity immunodominant epitope analog from human myelin basic protein (MBP), which shows release of gamma-IFN by specific stimulation of transformed human T cell clone (SS8T). The present report demonstrates a method for the localization of bound MHC class II-peptide complexes on T cell surface by backscatter electron imaging using in-lens Field Emission Scanning Electron Microscopy (FESEM). The localization is specific to the complex recognized by the TCR on MHC class II (DR2) and MBP peptide restricted human T cells.