Dirt and memory

eLife https://doi.org/10.7554/eLife.48901 (2019).

Immunological memory can develop even in the absence of overt infections; however, the nature and timing of signals that drive T cells to become either central memory (T_CM) or effector (T_EM) cells is unclear. Seddon and colleagues apply mathematical modeling based on quantitative comparisons of CD4⁺ T cells from conventionally housed mice (relatively ‘dirty’), ventilated-cage-housed mice, and in some cases germ-free mice (the latter two being relatively free of environmental antigens), to investigate memory formation. Modeling suggests that memory cells develop serially from naive to T_CM to T_EM cells rather than naive cells directly to T_EM cells. The level of ‘dirtiness’ increases the size of the memory pool during an early-life expansion, but by adulthood the rates of memory cell formation and turnover are equivalent in the different groups and therefore largely controlled independently of the microbial environment.