Thymocyte Sensitivity and Supramolecular Activation Cluster Formation Are Developmentally Regulated: A Partial Role for Sialylation

Abstract

TCR reactivity is tuned during thymic development. Immature thymocytes respond to low-affinity self-ligands resulting in positive selection. Following differentiation, T cells no longer respond to low-affinity ligands, but respond well to high-affinity (foreign) ligands. We show in this study that this response includes integrin activation, supramolecular activation cluster formation, Ca2+ flux, and CD69 expression. Because glycosylation patterns are known to change during T cell development, we tested whether alterations in sialylation influence CD8 T cell sensitivity to low affinity TCR ligands. Using neuraminidase treatment or genetic deficiency in the ST3Gal-I sialyltransferase, we show that desialylation of mature CD8 T cells enhances their sensitivity to low-affinity ligands, although these treatments do not completely recapitulate the dynamic range of immature T cells. These studies identify sialylation as one of the factors that regulate CD8 T cell tuning during development.

The Sialyltransferase ST3Gal-I Is Not Required for Regulation of CD8-Class I MHC Binding during T Cell Development

“…As reported previously (20,27,39), we find that these animals show normal representation of major thymocyte populations, but have a severe reduction in peripheral CD8 ϩ T cells (Fig. 3, A and B).…”

“…This susceptibility to cell death was lost as thymocytes matured to the CD8 SP stage, and it was proposed that regulation of CD8 noncognate binding by altered sialylation may be responsible for these changes in mature thymocyte sensitivity (38). In keeping with this last point, we and others showed that both enzymatic desialylation and ST3Gal-I deficiency can enhance sensitivity of mature T cells to low affinity peptide/MHC ligands (39,40). Furthermore, neuraminidase treatment can promote induction of apoptosis following T cell activation through the TCR (41).…”

“…Specifically, the removal of terminal sialic acid adducts was shown to dramatically enhance TCR-independent binding of CD8 to MHC class I (i.e., noncognate CD8 binding) (20,21). Desialylation was also shown to enhance T cell sensitivity to lower affinity TCR ligands (39,40). Furthermore, based on the data from Moody et al (20), it was proposed that sialylation of core-1 O-glycans catalyzed by ST3Gal-I was a critical element in the developmental regulation of CD8-class I binding.…”

Endosomal TLR signaling is required for anti-nucleic acid and rheumatoid factor autoantibodies in lupus

“…These findings, combined with normal TD humoral responses in 3d mice (this study) and the lack of T cell population changes in nonautoimmune 3d mice (21), suggest that 3d T cells compensate for the slightly reduced function of 3d APCs. Possible explanations for this are dynamic tuning of T cells for which a large number of different molecular mechanisms have been identified, including cell signaling feedback, level of CD5 expression, sialylation, and miRNA expression (36)(37)(38)(39)(40), or modification of the T cell receptor repertoire. We are currently generating T cell receptor transgenic 3d mice to address this issue.…”

Activation of Murine CD4+ and CD8+ T Lymphocytes Leads to Dramatic Remodeling ofN-Linked Glycans

“…Notably, the conversion of the peanut agglutinin (PNA) 5 high (PNA high ) phenotype of immature CD4/CD8 medullary thymocytes to the PNA low phenotype of the mature singlepositive CD8 and CD4 thymocytes results from conversion of the PNA ligand, the O-linked glycan Gal␤1-3GalNAc␣Thr/Ser, to its sialylated form, NeuAc␣2-3Gal␤1-3GalNAc␣Thr/Ser, that is no longer recognized by PNA due to increased expression of a sialyltransferase, ST3Gal I (1-4). In CD8 T cells, this glycosylation change reduces the affinity of CD8 for MHC class I, suggesting that sialylation of CD8 O-glycans modulates CD8 function during selection and maturation of CD8 T cells (5)(6)(7). Naive CD8 cells of ST3Gal I null mice that are constitutively PNA high undergo rapid apoptosis in the periphery, reducing the CD8 population to 10% of wild type (2).…”