Homeostatic expansion of T cells during immune insufficiency generates autoimmunity

Regulation of B Cell Differentiation and Plasma Cell Generation by IL-21, a Novel Inducer of Blimp-1 and Bcl-6

IL-21 is a type I cytokine whose receptor is expressed on T, B, and NK cells. Within the B cell lineage, IL-21 regulates IgG1 production and cooperates with IL-4 for the production of multiple Ab classes in vivo. Using IL-21-transgenic mice and hydrodynamics-based gene delivery of IL-21 plasmid DNA into wild-type mice as well as in vitro studies, we demonstrate that although IL-21 induces death of resting B cells, it promotes differentiation of B cells into postswitch and plasma cells. Thus, IL-21 differentially influences B cell fate depending on the signaling context, explaining how IL-21 can be proapoptotic for B cells in vitro yet critical for Ag-specific Ig production in vivo. Moreover, we demonstrate that IL-21 unexpectedly induces expression of both Blimp-1 and Bcl-6, indicating mechanisms as to how IL-21 can serve as a complex regulator of B cell maturation and terminal differentiation. Finally, BXSB-Yaa mice, which develop a systemic lupus erythematosus-like disease, have greatly elevated IL-21, suggesting a role for IL-21 in the development of autoimmune disease.

T follicular helper cells express a distinctive transcriptional profile, reflecting their role as non-Th1/Th2 effector cells that provide help for B cells

Effector T cell responses have long been viewed in the context of the Th1/Th2 paradigm. Recently, a third major subset of nonpolarized effector T cells that provides help to B cells has been identified. These T cells, termed T follicular helper (T(FH)) cells, home to the B cell areas of secondary lymphoid tissue, through interactions mediated via the chemokine receptor CXCR5 and its ligand CXCL13. Affymetrix microarrays were used to identify transcription factors, cytokines, and cell surface molecules that underlie the differentiation pathways and functional properties of the T(FH) subset. The transcriptional profile of human CXCR5(+) T(FH) cells was compared with that of Th1 and Th2 cells, which enabled the identification of numerous genes expressed preferentially by T(FH) cells, over the other effector subsets. Certain T(FH) genes were also expressed by B cells and thus appear to be particularly relevant for humoral immunity. Abs were used to confirm the expression of several factors. In particular, CD84 and CD200, the cytokine IL-21, and the transcription factor BCL6 were all strongly associated with T(FH) cells. Gene microarrays reveal a highly distinctive transcriptional profile for a third subset of effector T cells that differs markedly from Th1 and Th2 cells.

Driven to Autoimmunity

In the lymphoid system, T cells respond to space or under-crowding by dividing to maintain their numbers. In this issue of Cell, evidence is provided that this homeostatic proliferation, coupled with excess production of a cytokine, IL-21, is a key factor in susceptibility to autoimmune diabetes.