Abstract
A defining feature of the majority of B regulatory (Breg) cells described in mouse and man is their production of IL-10. However, other subsets of Breg, which act in an IL-10 independent manner, also exist. Here, we describe a distinct subset of Breg, defined by high levels of PD-L1, which can suppress inflammation.
We are able to describe the importance of PD-L1 expression on B cells in the context of humoral immunity and disease in both mouse and man. Previously, we have shown that PD-L1 on B cells is an important regulator of T follicular helper (TFH) cell activity and downstream antibody responses. Through the use of chimeric mice and transfer approaches of B cell populations with varying degrees of PD-L1 expression we now demonstrate that a PD-L1[hi] B cell subset dramatically suppressed humoral responses through attenuating the activation of T cells and antibody production, independent of other reported suppressor populations.
We further describe how PD-L1hi Breg are refractory to B cell depletion therapy (αCD20 mAb treatment). The suppressive capacity of these Breg was demonstrated to be dependent on PD-L1. The retention of PD-L1hi Breg, post B cell depletion therapy, was attributed to elevated expression of the B cell activating factor (BAFF) receptors BAFF-R, TACI and BCMA, which displayed enhanced uptake of BAFF compared to other B cell subsets. Blockade of BAFF-R rendered PD-L1hi B cells sensitive to B cell depletion therapy.
Together, our studies identify a previously unknown regulatory B cell that acts through the PD-1/PDL1 pathway in limiting the differentiation and function of TFH cells. It also demonstrates a novel mechanism by which B cell depletion therapy works, and thus provides insight into the dynamic control of humoral immune responses in both homeostasis and disease.