Signal peptide cleavage is essential for surface expression of a regulatory T cell surface protein, leucine rich repeat containing 32 (LRRC32)
- Equal contributors
1 Department of Dermatology, University Hospitals Case Medical Center and Case Western Reserve University, Cleveland, OH, 44106 USA
2 Department of Dermatology and Allergology, University of Szeged, Szeged, Hungary
3 Department of Dermatology, University of Yamanashi, Yamanashi, Japan
4 Statistical Sciences Core, Center For Clinical Investigation, Case Western Reserve University, Cleveland, OH, 44106 USA
5 VA Medical Center, Cleveland, OH, 44106 USA
6 Current Address: Ohio State University Dermatology, 2012 Kenny Road, Columbus, OH, 43221, USA
7 Current Address: Department of Dermatology, Indiana University School of Medicine, 550 N. University Blvd., Suite 3240, Indianapolis, IN, 46202, USA
BMC Biochemistry 2011, 12:27 doi:10.1186/1471-2091-12-27Published: 26 May 2011
Elevated numbers of regulatory T cells (Tregs) have been implicated in certain cancers. Depletion of Tregs has been shown to increase anti-tumor immunity. Tregs also play a critical role in the suppression of autoimmune responses. The study of Tregs 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 Tregs. However, there is limited information regarding the processing of LRRC32 or the regulatory phenotype and functional activity of Tregs expressing LRRC32.
Using naturally-occurring freshly isolated Tregs, we demonstrate that low levels of LRRC32 are present intracellularly prior to activation and that freshly isolated LRRC32+ Tregs are distinct from LRRC32- Tregs 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+ Tregs are more potent suppressors than LRRC32- Tregs.
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 Treg populations. In summary, understanding the processing and expression of LRRC32 may provide insight into the mechanism of action of Tregs and the refinement of immunotherapeutic strategies aimed at targeting these cells.