Immunological memory to Listeria monocytogenes in rodents. IV. Studies on origin and fate of tissue-positioned T memory cells.

In this report on memory T cells mediating anti-microbial resistance to Listeria monocytogenes (LM) it was analysed whether memory cells found in tissue during late-phase (e.g. 10-60 days after infection) are long-lived progeny of cells which settled in tissues during early phase (e.g. 4-10 days after infection), or whether they are short lived but constantly replaced from other sources of memory cells. The study provides evidence for both mechanisms. Transfer and parabiosis experiments as well as radiometric and autoradiographic studies suggested that early-phase cells give rise to late-phase memory cells in the extravascular compartment. These memory cells were shown to mediate resistance and respond to antigen in vitro. Mediators of resistance in the unstimulated peritoneal cavity during late-phase are long-lived. On the other hand, parabiosis studies suggested that late-phase resident peritoneal cells which mediate resistance and respond to antigen in vitro have in part arrived after the end of early phase. Such cells are found in low numbers in central lymph during late-phase. The simplest interpretation of these data is that LM-specific lymphoblasts spontaneously extravasate and settle in tissues as long-lived memory cells. Since the numbers of LM-specific lymphoblasts released from lymphoid tissue is highest during early phase, the majority of resident memory cells are progeny of early-phase lymphoblasts.