A Role for Lysosomal-Associated Protein

Mechanisms by which cell surface levels of the BCR are regulated remain largely unknown. We found that B cells lacking the lysosomal-associated protein transmembrane 5 (LAPTM5) expressed higher levels of cell surface BCR than did wild-type (WT) B cells after Ag stimulation in vitro and in vivo. In addition, LAPTM5-deficient mice contained an increased frequency of Ag-specific B cells and produced greater amounts of Abs than did WT mice after immunization with a T-dependent Ag. Adoptive transfer of LAPTM5-deficient B cells with WT T cells into RAG1-deficient mice revealed that the increased surface BCR levels and the enhanced B cell activation and Ab production were due to a B cell intrinsic defect. As they aged, the LAPTM5-deficient mice had increased titers of serum IgM and autoantibodies and immune complex deposition in the kidney. Immunoflu-orescent and biochemical analysis revealed that LAPTM5 physically interacted with the BCR complex and promoted its degradation in the lysosomal compartment in mouse B cells. These results demonstrate a role for LAPTM5 in the negative regulation of cell surface BCR levels and B cell activation. T he BCR is composed of Ag-binding membrane Ig non-covalently associated with the disulfide-linked signal-transducing Iga and Igb subunits (1–3). BCR signaling is strictly regulated both positively and negatively to allow delivery of physiologically appropriate signals yet to prevent overstimula-tion that can lead to B cell hyperactivation (4–6). Deficiency of positive regulators of BCR signaling, such as the CD19 coreceptor (7–9) or the Bruton's tyrosine kinase BTK (10, 11), can lead to impaired B cell responses and immunodeficiencies, whereas the absence of negative regulators, such as CD22 (12–14) or the Src homology region 2 domain-containing phosphatase 1 (15), can cause enhanced B cell activation and autoimmunity. Appropriate BCR signaling is thus crucial for the normal function of B cells and induction of an optimal humoral immune response. Recent studies suggest that BCR signaling is also influenced by the levels of cell surface BCR. For example, B cells with a mutated Iga (16) or Igb (17) ITAM had elevated BCR levels due to impaired BCR internalization. These B cells exhibited increased Ca 2+ flux in response to Ag stimulation, suggesting that the proximal BCR signaling was enhanced. Conversely, B cells with two Iga cytoplasmic domains had reduced surface BCR levels due to enhanced BCR internalization (18). These B cells exhibited decreased Ca 2+ flux and were hypo-responsive to Ag stimulation in vitro and …