Glycosylation changes in hFUT1 transgenic mice increase TCR signaling and apoptosis resulting in thymocyte maturation arrest

Abstract

Glycosylation of cell surface proteins is important in thymocyte maturation. In particular, the level of sialylation of key glycoproteins such as CD45 is believed to play a major role in regulating TCR signaling, adhesion and apoptosis of developing thymocytes. We show here that transgenic expression of human α1–2 fucosyltransferase (hFUT1) in mice resulted in a marked shift from sialylation to fucosylation of thymocyte glycoproteins. This was associated with a significant reduction in thymocyte number, an increased rate of apoptosis in double positive and single positive thymocytes, and a maturation arrest at TCR-dependent developmental transitions reminiscent of CD45 deficiency. Indeed, CD45RB dimerization was elevated in hFUT1 thymocytes, consistent with its hyposialylation, and there was a corresponding increase in phosphorylation of the TCR-associated protein Lck. However, contrary to the reduced TCR signaling in CD45 null mice, basal and stimulated TCR signaling was higher in hFUT1 thymocytes than in wild type thymocytes. Our results therefore demonstrate that aberrant expression of a single glycosyltransferase can profoundly affect thymopoiesis, although the relative involvement of CD45-dependent and -independent mechanisms is yet to be determined.

Introduction

Thymocyte maturation is characterized by changes in cell surface glycosylation, regulated by the expression of a panel of glycosyltransferases and the availability of their substrates. Precise mechanisms remain poorly understood, but glycosylation changes modify thymocyte migration, adhesion, signaling and apoptosis. Immature cortical thymocytes are hyposialylated and express the core 1 O-glycan Galβ1–3GalNAc-Ser/Thr (Thomsen–Friedenreich antigen or T-Ag), recognized by peanut agglutinin lectin (PNA) (Lowe, 2001). Further glycosylation of the T-Ag is determined by the balance between the core 2 O-glycan inducing glycosyltransferase β1–6 N-acetylglucosaminyltransferase (C2GnT) and α2–3 sialyltransferase 1 (ST3Gal-1) (Fig. 1). C2GnT adds a branched N-acetylglucosamine moiety to create a substrate for further elongation with lactosamine residues without affecting PNA binding. ST3Gal-1 sialylates and ‘caps’ the T-Ag abolishing PNA binding and preventing the addition of core 2 structures (Gillespie et al., 1993). The action of ST3Gal-1 predominates when both enzymes are expressed (Priatel et al., 2000). Elongated core 2 glycans are found predominantly on immature cortical thymocytes and activated T cells, interacting with endogenous lectins such as galectins, inducing apoptosis, and selectins via sialyl Lewis X motifs (Fig. 1) (Baum et al., 1995, Lowe, 2001). Short sialylated core 1 structures are found on mature medullary thymocytes and are not known to interact with endogenous lectins.

Capping of terminal sugars with the relatively large and negatively charged sialic acid moiety profoundly modulates the properties of thymocytes and T cells. In mice deficient for ST3Gal-1 (Priatel et al., 2000), the absence of sialylated core 1 structures and concomitant increase in core 2 O-glycans results in a decrease in number and enhanced apoptosis of peripheral CD8+ T cells. The importance of sialic acid in signaling via the T-cell receptor (TCR) has also been elucidated (Starr et al., 2003). Reducing sialic acid by either enzymatic removal with neuraminidase or by the deletion of ST3Gal-1 increased the sensitivity of the TCR to low affinity ligands. Removal of sialic acid may impact TCR signaling directly by enhancing the affinity/avidity of TCR/MHC interactions, or indirectly by disrupting/amplifying TCR signal cascades.

Engagement of the TCR results in a cascade of kinase and phosphatase activity including the phosphorylation of CD3ɛ and recruitment of ZAP-70. T cell signaling is in part controlled by the phosphatase activity of CD45 on the tyrosine kinase Lck. CD45 is a large (180–220 kDa) variably glycosylated transmembrane molecule expressed on all haematopoietic cells including thymocytes. Its ligands and precise role remain poorly defined, though it appears to regulate cell–cell interactions and TCR signal transduction (Hermiston et al., 2003). Different glycoforms of CD45 are expressed during thymocyte development (Spits, 2002). The large RABC isoforms are rich in O-glycosylation sites and serve as major substrates for various glycosyltransferases, producing heavily O-glycosylated glycoforms. The importance of CD45 in thymocyte development is demonstrated by the observation that mice deficient in CD45 expression display a marked maturation arrest, primarily at the double positive (DP) to single positive (SP) transition point, and have uncoupling of TCR signaling (Byth et al., 1996, Stone et al., 1997). Homodimerization of cell surface CD45 is a key mechanism whereby its regulatory effect on TCR signal transduction is modified. Due to heavy glycosylation causing steric inhibition, CD45RB does not form homodimers, thereby maintaining its cytoplasmic phosphatase activity (Xu and Weiss, 2002). In contrast, the smaller isoforms form homodimers which inhibit phosphatase activity by spatially denying access to the catalytic site (Majeti et al., 1998, Majeti et al., 2000). Homodimerization of the glycosylated isoforms is favored by hyposialylation and this process may also require binding of the extracellular galectin-1, with subsequent induction of thymocyte apoptosis (Perillo et al., 1995).

Hyposialylation is also observed with overexpression of α1–2 fucosyltransferase (Aubert et al., 2000, Zerfaoui et al., 2000). α1–2 fucosyltransferase competes with ST3Gal-1 and fucosylates terminal galactose molecules, promoting the formation of core 2 structures (Sepp et al., 1997). Our laboratory has previously generated mice with a hFUT1 transgene under the control of the mouse MHC Class I H-2K^b promoter, that express the human α1–2 fucosyltransferase (hFucT1) enzyme (Shinkel et al., 1997). These hFUT1 transgenic mice exhibit a complex phenotype including significant lymphopenia, thymocyte maturation arrest and development of spontaneous colitis (Brown et al., 2004). In this paper we demonstrate that hFUT1 thymocytes have increased fucosylation, reduced sialylation and increased core 2 O-glycan expression, with an arrest in maturation at the TCR-dependent double negative 3 (DN3) to DN4 and double positive (DP) to single positive (SP) transitions. Dimerization of glycosylated CD45 isoforms was observed; however in contrast to previously reported in vitro studies, this was associated with increased basal and stimulated TCR signal transduction. Apoptosis was increased at all thymocyte maturation stages and would appear to be the most likely cause of the profound lymphopenia. Our data suggest that in this animal model a reduction in sialylation promotes CD45 dimerization and that this is responsible, at least in part, for increased TCR signaling and apoptosis in thymocytes.