Although memory T cells respond more vigorously to stimulation and they are more sensitive to low doses of antigen than naive T cells, the molecular basis of this increased sensitivity remains unclear. We have previously shown that the T cell receptor (TCR) exists as different-sized oligomers on the surface of resting T cells and that large oligomers are preferentially activated in response to low antigen doses. Through biochemistry and electron microscopy, we now showed that previously stimulated and memory T cells have more and larger TCR oligomers at the cell surface than their naive counterparts. Reconstitution of cells and mice with a point mutant of the CD3ζ subunit, which impairs TCR oligomer formation, demonstrated that the increased size of TCR oligomers was directly responsible for the increased sensitivity of antigen-experienced T cells. Thus, we propose that an "avidity maturation" mechanism underlies T cell antigenic memory.

Antigenic T cell stimulation requires interaction between the TCR of the T cell and cognate peptide–MHC molecules presented by the APC. Although studies with TCR-specific Abs and soluble peptide–MHC ligands have shown that the TCR needs to be crosslinked by two or more ligands to induce T cell stimulation, it is not understood how several MHC molecules loaded with the cognate antigenic peptide can produce crosslinking under physiological conditions. We show at the molecular level that large clusters of cognate peptide–MHC are formed at the surface of murine professional and nonprofessional APCs upon virus infection and that these clusters impinge on the stimulatory capacity of the APC. These clusters are formed by tight apposition of cognate peptide–MHC complexes in a configuration that is compatible with simultaneous engagement of two or more TCRs. This suggests that physiological expression of Ag allows formation of multivalent ligands for the TCR that permit TCR crosslinking and T cell activation.