Itolizumab-induced antigenic modulation of CD6 inhibits T cell activity

Abstract

Abstract CD6 is an immunomodulatory receptor on T cells that promotes immune synapse formation, T-cell activation, and cell migration into tissues by binding activated leukocyte cell adhesion molecule (ALCAM). Excessive activation through the CD6-ALCAM pathway has been implicated in the pathogenesis of multiple autoimmune and inflammatory diseases. Hence, the ability to modulate the level of activation is beneficial to disease resolution. Itolizumab is a humanized anti-CD6 monoclonal antibody that is specific for the membrane distal domain 1 of CD6. Previously, itolizumab was believed to sterically hinder the CD6-ALCAM interaction, thereby blocking T-cell co-stimulation. Here we describe antigenic modulation as an additional mechanism whereby binding of itolizumab to CD6 induces proteolytic cleavage of the extracellular portion of CD6. Upon treatment with itolizumab, surface levels of CD6 decreased in a dose- and time-dependent manner, as monitored by flow cytometry using a noncompetitive anti-CD6 monoclonal antibody. This loss was inhibited in the presence of protease inhibitors and the decrease in surface levels of CD6 was accompanied by an increase in levels of soluble CD6 in the supernatant. Furthermore, we assessed the effect of surface levels of CD6 on the response of cells to T cell stimulation in the presence of ALCAM. CD6low cells showed reduced T-cell activity compared to CD6highcells as measured by cell surface activation makers, including CD25, CD69 and PD-1, and cytokine production. These findings demonstrate that CD6 is an important regulator of T-cell activity and that modulating surface levels of CD6 is an effective method for fine-tuning the activity of T-cells.