Background/Purpose:

Follicular helper T (Tfh) cells provide help to B cells in germinal centers (GCs) of secondary lymphoid organs (SLOs) following primary antigen (Ag) challenge, with formation of memory B and long-lived plasma cells. Memory B cells can be recalled in secondary responses upon Ag rechallenge, with generation of short-lived plasmablast or secondary GCs with further rounds of B cell maturation. Human memory B cells function better in vitro with T cell help; however, the nature of the T cells that provide such help is unclear, with the role of Tfh cells, if any, not known. Our goal is to identify and characterize human memory B helper T cells. 

Methods:

Tonsils were obtained from tonsillectomies (2-18 y) done at Yale New Haven Hospital, with cells used for flow cytometry, qPCR, and cytokine analyses. Autologous CD4 T cell and B cells were co-cultured for immunoglobulin (Ig) production, as a readout for T dependent B cell maturation. Confocal microscopy was performed to localize memory B cells and helper T cells.

Results:

We found that human Tfh cells, as defined by classical flow cytometry markers (CXCR5^hi PD-1^hi), can be subdivided into two groups based upon expression of P-selectin glycoprotein ligand-1 (PSGL-1), a cell surface protein that in synergy with the T cell homing marker (CCR7) dictates residence in the T cell zone of SLOs. PSGL-1^lo CXCR5^hi PD-1^hi cells reside in B cell follicles as Tfh cells, whereas we found by confocal microscopy that PSGL-1^hi CXCR5^hi PD-1^hi cells reside in T cell zones outside B cell follicles. We defined the latter PSGL-1^hi CXCR5^hi PD-1^hi cells as triple hi cells (Thi), finding that except for their location in T cell zones and PSGL-1 expression, they shared with GC-resident Tfh cells a similar surface marker profile. Yet, by contrast to Tfh cells, Thi cells expressed more CCR7 with lower expression of the follicular homing marker CXCR5, findings consistent with their T zone location. Bcl6, the master transcriptional regulator for Tfh cells, was expressed in both populations at equivalent levels, but expression of Blimp1, a regulator of effector T cell development, was higher in Thi cells. Tfh cells and Thi cells produced comparable levels of IL-21; however, Tfh cells produced more IL-4 while Thi cells exclusively produced IL-10, with the latter important in provision of B cell help. Memory B cells produced more Ig when co-cultured with Thi cells than other CD4 subsets, with such help contact dependent and reliant upon CD40L, IL-21, and IL-10; moreover, Thi cells preferentially associated with memory B cells as T-B cell couplets in flow cytometry compared to Tfh cells. Confocal microscopy showed co-localization of Thi cells and memory B cells at the T-B border. Thi cells were also found in human spleens.

Conclusion:

We have identified a population of B T helper T cells in SLOs that promote maturation of memory B cells in a cytokine- and contact-dependent manner, with localization at the border of the T cell zone and B cell follicles in SLOs. They are distinct from classical GC-resident Tfh cells, and are likely critical for antibody recall responses. Their further characterization should pave the way for new understanding of human immune memory responses in normal and autoimmune subjects.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/a-novel-subset-of-cd4-t-cells-that-provides-help-for-human-memory-b-cell-responses/