Targeted checkpoint control of B cells undergoing positive selection in germinal centers by follicular regulatory T cells

Follicular regulatory T cells (Tfr) can play opposite roles in the regulation of germinal center (GC) responses. Depending on the studies, Tfr suppress or support GCs and B cell affinity maturation. However, which factors determine positive versus negative effects of Tfr on the GC B cells is unclear. In this study we show that GC centrocytes that express MYC upregulate expression of CCL3 chemokine that is essential for both the positive and negative regulation of GC B cells by Tfr. B cell intrinsic expression of CCL3 contributes to Tfr-dependent positive selection of foreign Ag-specific GC B cells. At the same time, expression of CCL3 is critical for direct Tfr-mediated suppression of GC B cells that acquire cognate to Tfr nuclear proteins. Our study suggests that CCR5 and CCR1 receptors promote Tfr migration to CCL3 and highlights Ccr5 expression on Tfr subset that expresses Il10. Based on our findings and previous studies, we suggest a model of chemotactically-targeted checkpoint control of B cells undergoing positive selection in GCs by Tfr, where Tfr directly probe and license foreign antigen-specific B cells to complete their positive selection in GCs but, at the same time, suppress GC B cells that present self-antigens cognate to Tfr.

Methods
All flow data were collect based on the following procedures.

Single-cell suspensions from draining lymph nodes and spleens were prepared and filtered through a 70-µm nylon cell strainer (BD). Red blood cells were lysed. Cells were washed in FACS buffer (2% FBS, 1mM EDTA, 0.1% NaN3 in PBS) and followed by surface staining for the indicated markers for 20 min at 4℃.  For intracellular staining, after surface markers were stained, cells were fixed and stained by using regulatory T cell Staining Buffer Set (Thermo Fisher Scientific) according to the manufacturer’s instructions. All samples were acquired on a BD FACSCanto flow cytometer. For cell sorting, enriched B cells and T cells were incubated with antibodies in Sorting buffer (0.5% FBS and 2 mM EDTA in PBS) and were performed on a BD FACSAria III cell sorter. 

All flow data were analyzed with FlowJo (version 10.6.0) software.

VH186.2 sequence analysis of GC B cells based on the following protocol.

Genomic DNA was extracted using QIAamp DNA Micro Kit (Qiagen) from sorted NP-specific GC B cells (B220+ Fas+GL7+IgDloNP+). The isolated genomic DNA was used for PCR amplification of variable heavy chain region 186.2-joining heavy chain region 2 segments (VH186.2-JH2). PCR was performed using 25 ng of genomic DNA with a previously described semi-nested PCR protocol (refer paper Germinal Center Dynamics Revealed by Multiphoton Microscopy with a Photoactivatable Fluorescent Reporter). Briefly, two sequential rounds of PCR were performed using high-fidelity DNA polymerase (TAKARA) with the outer forward primers 5’-CATGGGATGGAGCTGTATCATGC-3’ and outer reverse primer 5’-CTCACAAGAGTCCGATAGACCCTG-3’ for 30 cycles 98 °C for 10 s, 55 °C for 15 s and 68 °C for 120 s, followed by the inner forward primers 5’-GGTGACAATGACATCCACTTTGC-3’ and inner reverse primer 5’-GACTGTGAGAGTGGTGCCTTG-3’ with the same conditions. Final blunt-ended PCR products were added an A-tailing and then subcloned into pGEM-T Easy vector (Promega). Single clones were sequenced using T7 primers.

Sequencing results were analyzed with IgBLAST (NCBI). Only the unique sequences (clones) that matched to VH186.2 gene (IgHV1-72*01) were counted.