Professor Harvard University, Massachusetts, United States
Introduction/Rationale: The phenomenon of albumin-hitchhiking has been exploited in vaccination strategies. We hypothesized that albumin mediated protein delivery may have evolved to facilitate the presentation of conformational determinants directly to B cells to facilitate B cell activation. However, the receptor that potentially targets albumin to lymph nodes and facilitates the presentation of intact proteins to B cells has hitherto not been identified.
Methods: Surface binding, specificity and saturability studies were performed on human and murine immune cells. Membrane proteins were extracted from human Ramos B cell membranes and affinity chromatography was performed using an HSA (human serum albumin)-Sepharose matrix. Mass spectrometry was performed on eluted proteins. CRISPR/Cas9 mediated gene ablation was used to establish the identity of the HSA-receptor (HSA-R). HEK 293T cells were transfected with the cDNA for the HSA-R. In vitro culture of murine dendritic cells with HSA and HSA-bound proteins and immunization of a range of genetically altered recipient mice was performed to establish the in vivo function of the HSA-R.
Results: We purified the HSA-R using affinity chromatography and validated its identity using gene ablation and overexpression studies. CRISPR/Cas9 mediated gene ablation and overexpression in HEK 293T cells helped establish that the HSA-R is both necessary and sufficient to exhibit albumin on the surface of antigen presenting cells for at least 15 hours. In vivo experiments have established that this receptor on dendritic cells facilitates potent T dependent germinal center-based B cell responses.
Conclusion: A newly discovered poorly endocytic but specific albumin receptor on antigen presenting cells is likely crucial for facilitating antigen presentation to antigen-specific B cells and enhancing T-B cell collaboration against bacterial lipoproteins and other albumin-binding microbial proteins.
