Tracking early T follicular helper cell differentiation in vivo.

T follicular helper (Tfh) cells provide essential help to B cells for the generation of high-affinity antibodies. These mechanisms provide the basis for the success of modern vaccines, but dysregulated Tfh cell responses are also linked to autoimmune diseases. In addition to their established role in driving humoral immunity, Tfh cells are gaining attention for their role in other processes of the adaptive immune system. For example, Tfh cells may serve as transitional differentiation intermediates during effector and memory T-helper cell differentiation and as a reservoir of HIV-infected cells. While B cells are required for the full maturation and maintenance of Tfh cell responses, they are dispensable for the initial induction of the Tfh cell phenotype, which occurs at the priming stage through interaction with dendritic cells. Nevertheless, the precise mechanisms of these early events during Tfh cell differentiation remain relatively unknown. Here, we describe a method for tracking early Tfh cell differentiation by following cell division kinetics and phenotypic changes of recently activated antigen-specific CD4(+) T cells in vivo. As an example, we use this method to visualize the requirements for T cell-expressed CD28 for the differentiation of CXCR5(+)Bcl6(+) Tfh cells.