Homeostatic proliferation and survival of naïve and memory T cells.
The immune system relies on homeostatic mechanisms in order to adapt to the changing requirements encountered during steady-state existence and activation by antigen. For T cells, this involves maintenance of a diverse repertoire of naïve cells, rapid elimination of effector cells after pathogen clearance, and long-term survival of memory cells. The reduction of T-cell counts by either cytotoxic drugs, irradiation, or certain viruses is known to lead to lymphopenia-induced proliferation and restoration of normal T-cell levels. Such expansion is governed by the interaction of TCR with self-peptide/MHC (p/MHC) molecules plus contact with cytokines, especially IL-7. These same ligands, i.e. p/MHC molecules and IL-7, maintain naïve T lymphocytes as resting cells under steady-state T-cell-sufficient conditions. Unlike naïve cells, typical "central" memory T cells rely on a combination of IL-7 and IL-15 for their survival in interphase and for occasional cell division without requiring signals from p/MHC molecules. Other memory T-cell subsets are less quiescent and include naturally occurring activated memory-phenotype cells, memory cells generated during chronic viral infections, and effector memory cells. These subsets of activated memory cells differ from central memory T cells in their requirements for homeostatic proliferation and survival. Thus, the factors controlling T-cell homeostasis can be seen to vary considerably from one subset to another as described in detail in this review.
Boyman
Onur
O
Division of Immunology and Allergy, University Hospital of Lausanne (CHUV), CH-1011 Lausanne, Switzerland. onur.boyman@chuv.ch
Létourneau
Sven
S
Krieg
Carsten
C
Sprent
Jonathan
J
D000818:Animals; D049109:Cell Proliferation; D002470:Cell Survival / Q000276:immunology; D006706:Homeostasis / Q000276:immunology*; D006801:Humans; D007156:Immunologic Memory / Q000276:immunology*; D008213:Lymphocyte Activation / Q000276:immunology; D018448:Models, Immunological; D011948:Receptors, Antigen, T-Cell / Q000276:immunology; D013601:T-Lymphocytes / Q000166:cytology / Q000276:immunology*
D011948:Receptors, Antigen, T-Cell