Transmembrane domain-mediated Lck association underlies bystander and costimulatory ICOS signaling

Abstract

The B7-family inducible costimulator (ICOS) activates phosphoinositide-3 kinase (PI3K) and augments calcium mobilization triggered by the T-cell receptor (TCR). We surprisingly found that the entire cytoplasmic domain of ICOS is dispensable for its costimulation of calcium mobilization. This costimulatory function relies on the unique transmembrane domain (TMD) of ICOS, which promotes association with the tyrosine kinase Lck. TMD-enabled Lck association is also required for p85 recruitment to ICOS and subsequent PI3K activation, and Lck underlies both the bystander and costimulatory signaling activity of ICOS. TMD-replaced ICOS, even with an intact cytoplasmic domain, fails to support T FH development or GC formation in vivo. When transplanted onto a chimeric antigen receptor (CAR), the ICOS TMD enhances interactions between T cells and antigen-presenting target cells. Therefore, by revealing an unexpected function of the ICOS TMD, our study offers a new perspective for the understanding and potential application of costimulation biology.

Selecting costimulatory domains for chimeric antigen receptors: functional and clinical considerations

“…1 Both ICOS and CD28 recruit PI3K via a cytoplasmic YMXM motif (YMNM in CD28 and YMFM in ICOS), ICOS costimulation leading to greater PI3K activity than CD28. 40 Furthermore, ICOS is unable to recruit Grb-2, which may account for the reduced IL-2 expression elicited by ICOS-compared to CD28-mediated costimulation. 40 Furthermore, ICOS is unable to recruit Grb-2, which may account for the reduced IL-2 expression elicited by ICOS-compared to CD28-mediated costimulation.…”

“…39 Unlike CD28, however, the TM domain of ICOS is sufficient to recruit the tyrosine kinase Lck and to augment TCR-triggered calcium mobilisation. 40 Furthermore, ICOS is unable to recruit Grb-2, which may account for the reduced IL-2 expression elicited by ICOS-compared to CD28-mediated costimulation. 41 While CD28 is critical for the initiation of an immune response, ICOS costimulatory signals appear particularly important at the time of secondary antigen exposure.…”

Inducible T‐cell co‐stimulator: Signaling mechanisms in T follicular helper cells and beyond

“…We also found that mutations of the membrane‐proximal KKKY motif weaken calcium flux independently of PI3K activation in Jurkat cells or primary CD4 T cells reconstituted with mutant constructs . However, mutation of any one of the three Lys residues into Ala within the KKKY motif reduced the level of ICOS on the cell surface, indicating a dual role of the KKKY motif in stabilizing ICOS surface expression and calcium flux . In contrast, Wan et al found that the ICOS TM segment and the first Lys residue in the KKKY motif are sufficient to maintain surface expression levels and induce calcium flux .…”

Evolution of chimeric antigen receptor (CAR) T cell therapy: current status and future perspectives

“…Interestingly however, recent studies have demonstrated that the sequence of the ICOS transmembrane domain is indispensable for inducing optimal costimulation signaling of the ICOS-based 2nd-generation CAR (vide infra) (Guedan et al 2014(Guedan et al , 2018Wan et al 2018). …”