How to measure human leukocyte antigen-specific B cells.

Purpose of review: The implementation of highly sensitive immune assays measuring anti-human leukocyte antigen (HLA) antibodies has modified alloimmune risk stratification and diagnosis of rejection. Nonetheless, anti-HLA antibodies represent the downstream effector mechanism of the B-cell response. Better characterizing the cellular components of the humoral immune response (including memory B cells (mBCs) and long-lived plasma cells) could help to further stratify the alloimmune risk stratification and enable discovery of new therapeutic targets. Several tests that characterize HLA-specific mBCs, either functionally or phenotypically, have been developed in the last years, showing promising applications as well as some limitations.

Recent findings: Functional assays involving ex vivo polyclonal activation of mBC have been refined to allow the detection of HLA-specific mBC capable of producing anti-HLA Abs, using different and complementary detection platforms such as multiplex Fluorospot and single antigen bead assay on culture supernatants. Detection of circulating HLA-specific B cells by flow cytometry remains hindered by the very low frequency of HLA-specific mBC.

Summary: Technological refinements have allowed the development of tests detecting HLA-specific mBC. Further evaluation of these assays in clinical trials, both for immune risk stratification and to assess treatment efficacy (desensitization strategies, rescue therapies for ABMR) are now urgently needed.