Amanda C. Poholek

T Follicular Helper Cell Differentiation When B cells respond to an infection, they often require help from CD4 + T cells to mount a proper response. It is thought that a subset of CD4 + effector T cells, called T follicular helper cells (T FH ), performs this function. Several subsets of effector CD4 + T cells arise, depending on the type of infection, which have distinct transcriptional programs driving their differentiation. Whether this is also the case for T FH cells has not been clear (see the Perspective by Awasthi and Kuchroo ). Nurieva et al. (p. 1001 , published online 23 July) and Johnston et al. (p. 1006 ; published online 16 July) now demonstrate that the transcription factor Bcl6 is both necessary and sufficient for T FH differentiation and subsequent B cell–mediated immunity, suggesting that it is a master regulator of this lineage. Johnston et al. also show that expression of Bcl6 and the transcription factor, Blimp-1, are reciprocally regulated in T FH cells and that, when ectopically expressed, Blimp-1 inhibits T FH development.

The role of specialized follicular helper T (T(FH)) cells in the germinal center has become well recognized, but it is less clear how effector T cells govern the extrafollicular response, the dominant pathway of high-affinity, isotype-switched autoantibody production in the MRL/MpJ-Fas(lpr) (MRL(lpr)) mouse model of lupus. MRL(lpr) mice lacking the Icos gene have impaired extrafollicular differentiation of immunoglobulin (Ig) G(+) plasma cells accompanied by defects in CXC chemokine receptor (CXCR) 4 expression, interleukin (IL) 21 secretion, and B cell helper function in CD4 T cells. These phenotypes reflect the selective loss of a population of T cells marked by down-regulation of P-selectin glycoprotein ligand 1 (PSGL-1; also known as CD162). PSGL-1(lo) T cells from MRL(lpr) mice express CXCR4, localize to extrafollicular sites, and uniquely mediate IgG production through IL-21 and CD40L. In other autoimmune strains, PSGL-1(lo) T cells are also abundant but may exhibit either a follicular or extrafollicular phenotype. Our findings define an anatomically distinct extrafollicular population of cells that regulates plasma cell differentiation in chronic autoimmunity, indicating that specialized humoral effector T cells akin to T(FH) cells can occur outside the follicle.