Impairment of immunological memory in the absence of MHC despite survival of memory T cells.

The mechanisms by which immunological memory is maintained after infection or vaccination are still a matter of debate. Long-term survival of memory T cells does not require major histocompatibility complex (MHC) contact. We show here that compared with memory CD4+ T cells that maintain contact with MHC class II, memory CD4+ T cells deprived of MHC class II contact show distinct functional defects upon antigen re-encounter. Thus, in contrast to their survival, maintenance of the typical quality of memory T cells crucially depends on MHC-derived signals.