This problem stems from several issues: first, BVD-523 nmr many of the markers used to identify Tfh cells, such as PD-1, ICOS and CXCR5, are also commonly expressed by activated CD4+ T cells.3,6,7 As a result, Tfh cells are often identified as the cells expressing the highest levels of these molecules; thus, it is easy to see how this can quickly become a problematic definition. Secondly, the term ‘Tfh cell’ is used by individual researchers to describe different populations of cells. Hence, while the original reports used the term to describe CD4+ CXCR5+ T cells located in the follicle, in more recent times ‘Tfh cell’ has come to be used by many to describe only those cells that

are found within learn more the germinal centre (GC), while CD4+ CXCR5+ T cells found elsewhere in the follicle are termed ‘pre-Tfh cells’. In contrast, others have maintained the usage of ‘Tfh cell’ to describe all CD4+ CXCR5+ T cells in the follicle and refer instead to those cells located specifically in the GC as ‘GC-Tfh cells’. Even given a consensus on the terminology for these cell populations, it remains to be determined whether follicular and GC-Tfh cells can be distinguished phenotypically or whether they can only be identified by imaging which reveals their location. Although some reports have suggested that molecules such as GL720 are able to identify specifically cells found

in the GC, other reports have suggested that at different times during the response, cells outside the GC can also express PI3K inhibitor these molecules.21 Once again, this probably reflects the problem that many markers of Tfh cells are also found on activated cells. The story is complicated further by recent reports that demonstrate that even Bcl-6, considered one of the gold standard markers of Tfh cells, cannot be used on its own to identify Tfh cells. These studies revealed that CD4+ T cells express Bcl-6 very quickly following

activation, long before they migrate deep into the follicle, let alone into the GC.21–23 Moreover, they identified cells with a Tfh-like phenotype (e.g. CXCR5 and PD-1 expression and GC localization) that did not express Bcl-6 as well as cells that expressed Bcl-6, but not other Tfh cell markers such as PD-1.21,22 This suggests that the role of Bcl-6 in regulating Tfh cell differentiation may be more complex than first anticipated. However, for the purposes of this review we will consider Tfh cells to be CXCR5+ PD-1+ Bcl-6+ cells that express IL-21 and are found in the follicle. A further problem has arisen in studies of human TFH cells, particularly in the investigation of patients suffering from immunodeficient or autoimmune conditions. In these patients it would be helpful to be able to identify Tfh cells to determine whether the generation or function of these cells is dysregulated.