T cells modulate glycans on CD43 and CD45 during development and activation, signal regulation, and survival

Abstract

Glycosylation affects many essential T cell processes and is intrinsically controlled throughout the lifetime of a T cell. CD43 and CD45 are the two most abundant glycoproteins on the T cell surface and are decorated with O- and N-glycans. Global T cell glycosylation and specific glycosylation of CD43 and CD45 are modulated during thymocyte development and T cell activation; T cells control the type and abundance of glycans decorating CD43 and CD45 by regulating expression of glycosyltransferases and glycosidases. Additionally, T cells regulate glycosylation of CD45 by expressing alternatively spliced isoforms of CD45 that have different glycan attachment sites. The glycophenotype of CD43 and CD45 on T cells influences how T cells interact with the extracellular environment, including how T cells interact with endogenous lectins. This review focuses on changes in glycosylation of CD43 and CD45 occurring throughout T cell development and activation and the role that glycosylation plays in regulating T cell processes, such as migration, T cell receptor signaling, and apoptosis.

Figure 1

Thymocytes and T cells express two glycoforms of CD43. The CD43 polypeptide backbone extends ~45 nm from the T cell surface and is decorated by ~80 O-linked glycans. Human CD43 also has one N-glycan site, which is not depicted in this schematic, as the significance of this glycan has not been determined. CD43 can be decorated by either sialylated core 1 O-glycans or core 2 O-glycans, generating the 115 kDa and the 130 kDa glycoforms of CD43, respectively. The 115 kDa glycoform of CD43 is the primary glycoform expressed by immature double negative thymocytes, mature CD4 or CD8 single positive thymocytes, and naïve peripheral T cells. The 130 kDa glycoform of CD43 is upregulated by immature double positive thymocytes and by activated peripheral T cells.

Figure 2

T cells decorate their glycoproteins with asialo-core 1, sialylated core 1, and core 2 O-glycans and high mannose-, hybrid-, and complex-type N-glycans. (A) Core 1 structures can be uncapped (asialo) or α2,3-sialylated by the enzyme ST3Gal1. Core 1 O-glycans can also be extended into core 2 O-glycans by the enzyme C2GnT. Core 2 O-glycans contain lactosamine (Gal-GlcNAc) sequences and may or may not also be capped by ST3Gal1 (this schematic depicts uncapped core 2 O-glycans). (B) High mannose N-glycans are precursors for hybrid-type and complex-type N-glycans. Complex N-glycans on T cells are elaborated by the action of the enzyme GnT-V, which adds a β1,6-linked GlcNAc residue that can be further extended into a polylactosamine sequence (bracket). Complex N-glycans can be capped by α2,3- or α2,6-linked sialic acid.

Figure 3

There are five isoforms of CD45 on human lymphocytes due to alternative splicing. Intracellularly, all isoforms of CD45 have tandem phosphatase domains (P1 and P2; only P1 is enzymatically active). Extracellularly, all isoforms have three fibronectin type III repeats (F), a cysteine-rich region (CR), and a terminal region. The F, CR, and terminal regions are the primary sites of N-linked glycosylation. Alternative splicing of exons 4, 5, and 6 generates up to three additional extracellular domains—termed A, B, and C, respectively—and a total of five isoforms of CD45: RO (with no alternatively spliced domains), RB (exon 5), RA (exon 4), RBC (exons 5 and 6), and RABC (exons 4-6). The extended region and alternatively spliced regions are the primary sites of O-linked glycosylation, although RA and RC also have canonical sites of N-linked glycosylation. The schematics of the five isoforms of CD45 depict glycans based on the following assumptions: 1) The molecular weights of CD45RO, RB, RA, RBC, and RABC are 180, 190, 210, and 230, respectively; 2) Each N-glycan is complex and contributes ~4 kDa to the overall molecular weight; RO has core 2 O-glycans, each contributing ~1.5 kDa to the overall molecular weight; RB, RA, and RBC have sialylated core 1 O-glycans, each contributing ~0.7 kDa to the overall molecular weight; 3) All N-glycan sites in the F, CR, and extended regions are occupied in all isoforms; 4) Some O-glycan sites may overlap between isoforms, but the total number of O-glycans should be higher for larger isoforms (i.e., RO should have the least number of O-glycans, while RABC should have the most O-glycans); 5) Domains with more serine and threonine residues should have more potential O-glycosylation sites (i.e., RB has the least and RA has the most); and 6) The alternatively spliced domains A, B, and C should have O-glycan occupancy approximately proportional to the number of serine and threonine residues contained in the polypeptide sequences.