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 (Tregs) have been implicated in certain cancers. Depletion of Tregshas been shown to increase anti-tumor immunity. Tregsalso play a critical role in the suppression of autoimmune responses. The study of Tregshas 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 Tregs. However, there is limited information regarding the processing of LRRC32 or the regulatory phenotype and functional activity of Tregsexpressing LRRC32. Results: Using naturally-occurring freshly isolated Tregs, we demonstrate that low levels of LRRC32 are present intracellularly prior to activation and that freshly isolated LRRC32+ Tregsare distinct from LRRC32- Tregswith 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+ Tregsare more potent suppressors than LRRC32- Tregs. 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 Tregpopulations. In summary, understanding the processing and expression of LRRC32 may provide insight into the mechanism of action of Tregsand the refinement of immunotherapeutic strategies aimed at targeting these cells. © 2011 Chan et al; licensee BioMed Central Ltd.