(237) Antibodies cloned from a patient with Idiopathic CD4 Lymphopenia induce potent anti-T and B cell complement-dependent lysis

Introduction/Rationale: Idiopathic CD4 Lymphopenia (ICL) is a rare immunodeficiency disorder characterized by an unexplained CD4+ T-cell lymphopenia. Often, these patients harbor additional deficiencies in CD8, B, and/or NK cells and, therefore, may suffer from opportunistic infections. We previously found that half of patients with ICL possess auto-antibodies-(Ab) against a broad spectrum of molecules, including those expressed on the membranes of B and T-cells, suggesting a potential role of these anti-lymphocyte Ab (ALAb) in their lymphopenia/s. We focused here on a patient (ICL-9) whose IgM ALAb induced complement dependent cytotoxicity (CDC) against healthy donor’s (HD) lymphocytes and against B-cell lymphoma cell lines.

Methods: We performed single-cell B-cell receptor (BCR) sequencing of sorted ICL-9’s B cells, cloned 21 IgM BCR sequences, and screened them for specificity against B/T lymphocytes and Raji B-cell line. The positive reactive clones were further tested for CDC of their cellular targets.

Results: The cloned ALAb belonged to the previously described autoreactive VH4-34 family. Ten of the auto-Ab recognized B and/or T cells, with three of them inducing lysis of their targets by CDC. A particularly potent clone, named Aliron, induced CDC of 50% of B lymphocytes at 0.01 nM concentration, showing a 100-fold higher CDC potency than anti-CD20 Ab Rituximab. At higher concentrations, Aliron was also able to kill T-cells by CDC. Further, Aliron induced CDC of all seven B-cell lymphoma lines tested. We are currently analyzing the in vivo activity of these IgM monoclonal Abs in humanized mouse models.

Conclusion: From a single patient with ICL we cloned and characterized IgM Abs that recognized membrane targets on T and B lymphocytes and induce CDC. The discovery of these ALAb support their possible pathogenic role on ICL and they might develop into potential future clinical applications.