Signaling Lymphocytic Activation Molecule (SLAM)/SLAM-associated Protein Pathway Regulates Human B-cell Tolerance

Background: Signaling lymphocytic activation molecule (SLAM)-associated protein (SAP) can mediate the function of SLAM molecules, which have been proposed to be involved in the development of autoimmunity in mice.

Objective: We sought to determine whether the SLAM/SAP pathway regulates the establishment of human B-cell tolerance and what mechanisms of B-cell tolerance could be affected by SAP deficiency.

Methods: We tested the reactivity of antibodies isolated from single B cells from SAP-deficient patients with X-linked lymphoproliferative disease (XLP). The expressions of SAP and SLAM family members were assessed in human bone marrow-developing B cells. We also analyzed regulatory T (Treg) cell function in patients with XLP and healthy control subjects.

Results: We found that new emigrant/transitional B cells from patients with XLP were enriched in autoreactive clones, revealing a defective central B-cell tolerance checkpoint in the absence of functional SAP. In agreement with a B cell-intrinsic regulation of central tolerance, we identified SAP expression in a discrete subset of bone marrow immature B cells. SAP colocalized with SLAMF6 only in association with clustered B-cell receptors likely recognizing self-antigens, suggesting that SLAM/SAP regulate B-cell receptor-mediated central tolerance. In addition, patients with XLP displayed defective peripheral B-cell tolerance, which is normally controlled by Treg cells. Treg cells in patients with XLP seem functional, but SAP-deficient T cells were resistant to Treg cell-mediated suppression. Indeed, SAP-deficient T cells were hyperresponsive to T-cell receptor stimulation, which resulted in increased secretion of IL-2, IFN-γ, and TNF-α.

Conclusions: SAP expression is required for the counterselection of developing autoreactive B cells and prevents their T cell-dependent accumulation in the periphery.