Durable CD4+ T cell immunity: cherchez la stem.

Different CD4+ T cell subsets have been described to have 'stem-like' features, frequently without a clear indication of what that means functionally or devoid of a systematic set of criteria for how to apply the terminology. Bona fide stem cells are themselves varied and their definitions have been evolving recently. CD4+ T cell types with clear stem-like features are naive cells and subsets of memory cells [central memory (Tcm) and stem cell memory (Tscm)]. These also display stability over time, specific niche homing, high replicative potential, and the capacity to differentiate into many other T cell types. Other cells fall along a continuum of more versus less stem-like features. In some cases, more functional characterization of these cells is required before appending a 'stem-like' moniker is justified. For others, substituting 'progenitor' or 'precursor' terminology along the lines of the hematopoietic system is likely to be more accurate. T cells with stem-like features mediate durable immunity to pathogens and tumors but can also promote tissue damage. The complexity of the literature labeling T cells as 'stem-like' suggests that both the criteria for ascribing stem cell character to T cells, and the nomenclature used to relate them to one another, need refinement. This would help systemize the field, allowing better inference of function from name. Mammalian stem cells govern development, tissue homeostasis, and regeneration. Following years of study, their functions have been delineated with increasing precision. The past decade has witnessed heightened widespread use of stem cell terminology in association with durable T cell responses to infection, antitumor immunity, and autoimmunity. Interpreting this literature is complicated by the fact that descriptions are diverse and criteria for labeling 'stem-like' T cells are evolving. Working under the hypothesis that conceptual frameworks developed for actual stem cells can be used to better evaluate and organize T cells described to have stem-like features, we outline widely accepted properties of stem cells and compare these to different 'stem-like' CD4+ T cell populations. [ABSTRACT FROM AUTHOR]