Signal peptide cleavage is essential for surface expression of a regulatory T cell surface protein, leucine rich repeat containing 32 (LRRC32)

Abstract

Background: Elevated numbers of regulatory T cells (T(regs)) have been implicated in certain cancers. Depletion of T(regs) has been shown to increase anti-tumor immunity. T(regs) also play a critical role in the suppression of autoimmune responses. The study of T(regs) has been hampered by a lack of adequate surface markers. Leucine Rich Repeat Containing 32 (LRRC32), also known as Glycoprotein A Repetitions Predominant (GARP), has been postulated as a novel surface marker of activated T(regs). However, there is limited information regarding the processing of LRRC32 or the regulatory phenotype and functional activity of T(regs) expressing LRRC32. Results: Using naturally-occurring freshly isolated T(regs), we demonstrate that low levels of LRRC32 are present intracellularly prior to activation and that freshly isolated LRRC32+ T(regs) are distinct from LRRC32- T(regs) with respect to the expression of surface CD62L. Using LRRC32 transfectants of HEK cells, we demonstrate that the N-terminus of LRRC32 is cleaved prior to expression of the protein at the cell surface. Furthermore, we demonstrate using a construct containing a deleted putative signal peptide region that the presence of a signal peptide region is critical to cell surface expression of LRRC32. Finally, mixed lymphocyte assays demonstrate that LRRC32+ T(regs) are more potent suppressors than LRRC32- T(regs). Conclusions: A cleaved signal peptide site in LRRC32 is necessary for surface localization of native LRRC32 following activation of naturally-occurring freshly-isolated regulatory T cells. LRRC32 expression appears to alter the surface expression of activation markers of T cells such as CD62L. LRRC32 surface expression may be useful as a marker that selects for more potent T(reg) populations. In summary, understanding the processing and expression of LRRC32 may provide insight into the mechanism of action of T(regs) and the refinement of immunotherapeutic strategies aimed at targeting these cells.