CD43 Regulation of T Cell Activation Is Not through Steric Inhibition of T Cell–APC Interactions but through an Intracellular Mechanism

Abstract

CD43 is a large heavily glycosylated protein highly expressed on T cells and actively excluded from the immunological synapse through interactions with ezrin-radixin-moesin proteins. Due to its size and charge, it has been proposed that the CD43 ectodomain acts as a physical barrier to T cell–APC interactions. We have addressed this hypothesis by studying the effect of reconstituting CD43 mutants into the hyperproliferative CD43−/− T cells. Reintroduction of full-length CD43 reversed the CD43−/− T cell hyperproliferation. Interestingly, despite the lack of exclusion from the interaction site, a mutant containing the CD43 ectodomain on a glycosylphosphatidylinositol linkage was ineffective. Additionally, T cell–APC conjugate formation was not affected by this ectodomain-only construct. In contrast, CD43−/− T cell hyperproliferation was reversed by an intracellular-only CD43 fused to the small ectodomain of hCD16. Mutation of this intracellular-only CD43 such that it could not move from the T cell–APC contact site had no further affect on proliferation than the moveable CD43 but did dramatically reduce interleukin-2 production. Thus, the exclusion of the CD43 intracellular region from the immunological synapse is required for CD43 regulation of interleukin-2 production, but the presence of the cytoplasmic tail, independent of its location, is sufficient to reverse CD43−/− T cell hyperproliferation.

Inhibition of T Cell Activation by Cyclic Adenosine 5′-Monophosphate Requires Lipid Raft Targeting of Protein Kinase A Type I by the A-Kinase Anchoring Protein Ezrin

“…However, recent data support that it is rather the intracellular domain of CD43 that is involved in the negative regulation of T cell responsiveness because expression of a CD43-extracellular domain fusion protein failed to reverse the hyperactivated state of CD43 null mutant mice. In support of this, reversal of the hyperactivated state was achieved with a fusion protein containing the intracellular domain of CD43 and the transmembrane domain of CD7 (61). These data argue against the barrier hypothesis.…”

P-Selectin Glycoprotein Ligand-1 Negatively Regulates T-Cell Immune Responses

“…In support of this finding, two groups have published data demonstrating that a CD43 mutant lacking the intracellular domain is ineffective in reverting the hyperproliferative phenotype of CD43 Ϫ/Ϫ T cells (28,31). Although the intracellular domains of PSGL-1 and CD43 have no obvious homology, this domain is known to be involved in signal transduction.…”

T-cell-receptor-dependent actin regulatory mechanisms

“…The composition of the DPC indicates that it may serve as a sink for negative regulators of activation events occurring at the IS. CD43 negatively regulates T-cell activation, and expression of a CD43 mutant that fails to be sequestered in the DPC because it cannot interact with ERM proteins leads to further suppression of IL-2 production (Tong et al, 2004). Rho-GDI can inhibit RhoGTPase activation, and its sequestration in the DPC could therefore facilitate activation of Rho-GTPases at the IS.…”