Germinal centers continually recruit naïve T cells that alter the balance between follicular T helper  and follicular regulatory T cells.
Follicular helper T cells (T FH ) mediate B cell selection and clonal expansion in germinal centers (GCs), and follicular regulatory T cells (T FR ) prevent the emergence of self-reactive B cells and help to extinguish the reaction. Here we show that GC reactions continually recruit T cells from both the naïve conventional and naive thymic regulatory T cell (Treg) repertoires. In the early GC, newly recruited T cells develop into T FH , whereas cells entering during the contraction phase develop into T FR cells that contribute to GC dissolution. The T FR fate decision is associated with decreased antigen availability and is modulated by slow antigen delivery or mRNA vaccination. Thus, invasion of ongoing GCs by newly developing T FH and T FR from naïve conventional and Treg repertoires, helps remodel the GC based on antigen availability. Overall design: For single cell RNA sequencing, single cell suspensions were prepared from half-spleens of NP-OVA immunized SellCreERT2 ROSAtdT mice on day 7 and 21 after immunization. Samples were indexed with TotalSeqC (BioLegend) cell surface antibodies and CD4 + , CD62 low , CD44 hi , PD1 hi , CXCR5 high , tdTomato + Tfh cells were purified by flow cytometry, pooled and loaded onto a Chromium Controller (10x Genomics). Single-cell RNA-seq libraries were prepared using the Chromium Single Cell 5' v2 Reagent Kit (10x Genomics) according to manufacturer's protocol. Libraries were loaded onto an Illumina NextSeq with the mid-Output Kit (150 paired end) for V-D-J analysis or NOVAseq for single cell gene expression. Hashtag indexing was used to demultiplex the sequencing data and generate gene- barcode matrices, respectively.