Cellular & Molecular Immunology (2013) 10, 379–382 ß 2013 CSI and USTC. All rights reserved 1672-7681/13 $32.00 www.nature.com/cmi RESEARCH HIGHLIGHT Immune regulation by CD52-expressing CD4 T cells Ban-Hock Toh1, Tin Kyaw1,2, Peter Tipping1 and Alex Bobik2 T-cell regulation by CD52-expressing CD4 T cells appears to operate by two different and possibly synergistic mechanisms. The first is by its release from the cell surface of CD4 T cells that express high levels of CD52 that then binds to the inhibitory sialic acid-binding immunoglobulin-like lectins-10 (Siglec-10) receptor to attenuate effector T-cell activation by impairing phosphorylation of T-cell receptor associated lck and zap-70. The second mechanism appears to be by crosslinkage of the CD52 molecules by an as yet unidentified endogenous ligand that is mimicked by a bivalent anti-CD52 antibody that results in their expansion. Cellular & Molecular Immunology (2013) 10, 379–382; doi:10.1038/cmi.2013.35; published online 12 August 2013 he immune system is designed to appears in the affirmative, and includes suppression was lost by cleavage of N- T protect its host from invading players such as IL-10-secreting Tr1 and glycans from CD52-Fc by peptide N- pathogens and yet remain non-reactive TGF-b-secreting Th3. cells. Absence of glycosidase or by removal of sialic acid to self. Immunological homeostasis is surface markers limited the usefulness residues by neuraminidase. Suppression maintained by purging self-reactive lym- of these other regulators. However, the was also blocked by antibody to the phocytes by clonal deletion coupled with recent report that CD49b and lympho- extracellular domain of Siglec-10 and a regulatory population of lymphocytes cyte activation gene-3 are highly and sta- by soluble Siglec-10-Fc. Given that that suppress self-reactive lymphocytes bly coexpressed by human and mouse Siglec-10 contain two immunoreceptor that have ‘escaped’ clonal deletion. Tr1 has provided an approach for studies tyrosine-based inhibition motifs, it Disruption of the delicate balance of of IL-10-secreting Tr1 cells.2 seems likely that recruitment of SHP regulatory and effector lymphocytes, is Yet another T-cell regulator has phosphatases by these motifs impaired thought to be a major contributor to recently gained prominence by a recent phosphorylation of Lck and Zap-70. the development of autoimmune dis- report in Nature Immunology of CD4 T The observation of a lower frequency ease. Pioneering studies of Shimon cells expressing high levels of CD52.3 and impaired function of these regula- Sakaguchi have led to the firm estab- CD52 is a short peptide of 12 amino tors in human type 1 diabetes coupled lishment of CD251Foxp3 expressing acids in humans linked at its C-terminus with the aggravated diabetes following CD4 T lymphocytes as fundamental for to a glycosylphosphatidylinositol (GPI) transfer of lymphocytes deleted of this immune homeostasis as the loss of this anchor. Its single N-glycosylation site is population suggest that these may pro- population results in intractable auto- occupied by large sialylated polylactossa- tect from the development of auto- immune disease in man and mouse.1 mine on a tetra-antennary fucosylated immune disease. While a loss of The transcription factor Foxp3 plays a mannose core.4 Suppression appears to function of CD52 was demonstrated in key role in their development. These cells be mediated by soluble CD52, released the mouse model of type 1 diabetes, a are a distinct CD4 T-cell lineage that from its GPI anchor by phospholipase gain of function with augmented CD52 arise in the thymus, but can also be gen- C, which then binds to inhibitory sialic ameliorating disease was not explored. erated in the periphery. acid-binding immunoglobulin-like lec- Also endogenous molecular mechanisms But are there other players in main- tins-10 (Siglec-10) receptor to inhibit that activate cleavage of the GPI anchor taining T-cell homeostasis? The answer T-cell activation by impairing phosphor- of CD52 to release it in soluble form ylation of T-cell receptor associated remains unknown. 1Centre for Inflammatory Diseases, Department kinases Lck and Zap70 (Figure 1). Siglec-10 was cloned from a human of Medicine, Southern Clinical School, Faculty of Siglec-10 belongs to a major subfamily dendritic cell library in 20017 and was Medicine, Nursing and Health Sciences, Monash University, Clayton, Vic., Australia and 2Vascular of Ig-like lectins that mediate sialic acid also identified by database mining of an 8 Biology and Atherosclerosis Laboratory, Baker recognition by their terminal Ig like asthmatic eosinophil EST library. It is IDI Heart and Diabetes Institute, Melbourne, domains5 to regulate innate and adaptive expressed in peripheral blood mononuc- Vic., Australia 6 Correspondence: Ban-Hock Toh immune responses. The data suggest lear cells including monocytes, dendritic E-mail: [email protected] that sialic acid structures of CD52 binds cells and B lymphocytes. Given its broad Received: 1 July 2013; Accepted: 2 July 2013 to Ig-like domain of Siglec-10 because immune distribution, suppression by Research Highlight 380 CD52 CD52 Siglec-10 Phospholipase P Zap 70 CD52hiCD52 hi CD4+ T cell CD52low LCK CD4+ T cells TCR Figure 1 T-cell regulation by soluble CD52 binding to Siglec-10. CD52 release from its GPI anchor by phospholipase binds via its terminal sialic acid domain to the terminal Ig domain of Siglec-10. Phosphatase activation by the ITIM motifs of Siglec-10 impairs phosphorylation of T-cell receptor associated lck and zap70. GPI, glycosylphosphatidylinositol; ITIM, immunoreceptor tyrosine-based inhibition motif; Siglec-10, sialic acid- binding immunoglobulin-like lectins-10. CD52 released from CD52-expressing stimulation.12 Co-injection of regulatory conventional CD4 T cells and there are regulatory CD4 T cells may not be T cells expanded in culture by IL-2 multiple isoforms of this molecule, it has restricted to T cells only, but may also and anti-CD52 suppressed lethal graft been suggested that the antibody that extend to a broader population of peri- vs. host disease in severe combined costimulates regulatory T-cell expansion pheral blood mononuclear cells that immunodeficiency disorder recipients may bind to an epitope preferentially include dendritic cells, monocytes and caused by human peripheral blood expressed on regulatory T cells. B lymphocytes. As human Siglec-10 also mononuclear cells. As 4C8 is an IgG3 CD52-expressing CD4 T cells differ binds to vascular adhesion protein-1 to monoclonal antibody, it probably acts from CD251Foxp31 regulatory CD4 T act as its substrate and to mediate as a costimulator by crosslinking CD52 cells in a number of important aspects. lymphocyte binding to the endothe- molecules on the cell surface, to expand It differs in its distribution in that CD52 lium,9 binding of CD52 to siglec-10 CD4 T cells that suppresses in a contact- is also found in sperm as its major matu- could also potentially modify the inflam- dependent manner (Figure 2). The epi- ration-associated membrane protein.16 matory microenvironment. tope for this antibody as with the It is also more widely distributed in the The CD52 story may be even more CAMPATH antibody13 likely lies on an immune system as it is expressed by all complex than revealed by the above stu- amino-acid sequence proximal to the lymphocytes, monocytes and dendritic dies. Earlier reports suggest that CD52 terminal sialic acids that bind to the ter- cells.17,18 It is also expressed by eosino- recognized by monoclonal antibody minal Ig domain of Siglec-10. A natural phils but not by neutrophils.18 Despite its 4C8 can also act as a novel costimulatory ligand that crosslinks CD52 remains expression in lymphocytes and sperm molecule for the induction of CD4 re- unknown. Nonetheless the situation here and the report that the N-glycans of a gulatory T cells. The 4C8 IgG3 monoclo- is analogous to the costimulatory expan- CD52 isoform is associated with human nal antibody was first reported for its sion of CD41Foxp31 regulatory T cells infertility,19 CD52 gene knockout gene- capacity to inhibit the post-adhesive by monoclonal antibody to the tumor- rates no discernable phenotype in the transendothelial migration of T cells necrosis factor (TNF) receptor super- immune and reproductive systems.20 through human endothelial cell mono- family member 25 that is constitutively This stands in contrast to the fatal auto- layers.10 In a subsequent study, CD4 T and highly expressed by these regulatory immune disease phenotype arising from cells expanded by stimulation with T cells.14 The expansion, dependent on the phenotype arising from knockout of anti-CD3 and costimulation with anti- T-cell receptor engagement with MHC CD2521 or of the Foxp3 transcriptional 4C8 expressed high levels of IL-2 that class II antigen, prevents allergic lung regulator.1,22 The developmental path- suppressed in a contact-dependent man- inflammation. The TNF receptor super- way of these T cells in the thymus and/ ner.11 The suppressor cells were shown to family member 25 ligand in this instance or in the periphery is also not known. develop from CD41CD252CD45RO1 has been identified as sTLIA, the con- CD52 is also the molecular target of precursor cells. These anti-CD52 stitutive expression of which in trans- CAMPATH-1, a rat monoclonal anti- induced regulatory T cells suppressed genic mice expands these regulatory T body raised against human lymphocyte proliferation of CD4 and CD8 T cells cells.15 Given that TNF receptor super- proteins by Hermann Waldmann in the provided with polyclonal or allogeneic family member 25 is also expressed on Cambridge Pathology Department.23 Cellular & Molecular Immunology Research Highlight 381 Characterization of the 4C8 antigen involved in transendothelial migration of CD26hi T CD52 cells after tight adhesion to human umbilical 4C8 mAb vein endothelial cell monolayers.