CD4 and infection: Previous studies provided anecdotal evidence that the longevity of S. Typhimurium-specific CD4+ T cells may depend on how well their cognate antigens can persist in vivo: for instance, CD4+ T cells recognising flagellin, which is rapidly downregulated within hours of infection, showed premature contraction prior to bacterial clearance and formed poor memory [20,61,62]; whereas CD4+ T cells recognising SseJ, a Salmonella Pathogenicity Island-2 (SPI-2) effector protein with sustained expression, were maintained at a stable level longer term [20,63].