(A). Confocal image of a spontaneous GCs result from interactions of CD4+ Tfh B cells that express Bcl6 and a separate population of CD4 T cells that express IL-17 in the LZ of the GC. The representative BCL6+ GC (red) is derived from the spleen of a BXD2 mouse. The GC is simultaneously stained with an anti-mIL-17 (white) and an anti-mCD4 (green) to illustrate the distribution of Tfh-IL-17+ cells in the GC (objective lens 20×). (B) In the spleen, development of mixed populations of Tfh occur at the late naïve stage (CD62L+CD44+, Population C) when there is high expression of IL-21, IL-17, CXCR5 and ICOS. This population has undergone approximately 4 cell divisions as indicated by a quantitative real-time PCR-based TREC assay(148), and expresses the highest levels of IL-21 or IL-17 as analyzed by intracellular flow cytometry(44).