ICOS

Overview

ICOS (Inducible T-cell COStimulator, CD278) is a co-stimulatory receptor of the CD28 superfamily expressed on activated T cells. ICOS signalling through its ligand ICOS-L (B7-H2) is critical for T follicular helper (TFH) cell differentiation, germinal center maintenance, and T cell-dependent antibody responses. In flow cytometry panels, ICOS expression on CD4⁺ T cells is used as a marker of TFH activation.

Key Points from Literature

• CD4⁺ICOS⁺ TFH cells diminished in COVID-19 lymphoid tissue: Multi-color immunofluorescence of post-mortem COVID-19 lymph nodes and spleens showed diminished CD4⁺ICOS⁺ TFH cells compared with non-COVID controls. This reduction was part of a broader TFH differentiation block: CD4⁺CXCR5⁺ pre-GC TFH cells were present but reduced, and CD4⁺Bcl-6⁺ GC-type TFH cells were near-absent. ICOS⁺ TFH depletion occurred alongside TH1 (T-bet⁺) expansion and aberrant TNF-α accumulation (see Kaneko2020 - GC Loss and TFH Block in COVID-19, n=11 COVID + controls, multi-color immunofluorescence).

• ICOS-dependent Tfh differentiation is required for the murine T-bet⁺CD11c⁺ B cell response. Icos⁻/⁻ mice — which have defective Tfh differentiation and more Th1 cells than wild type — showed reduced frequencies and numbers of T-bet⁺CD11c⁺ B cells at day 10 p.i. (see Song2022 - Tfh Outside Germinal Centers Drive T-bet CD11c B Cells, mouse, LCMV-Armstrong, n=3–5 mice/group). The Th1 excess in the same animals is what makes this a Tfh attribution rather than a generic loss of T cell help, and it is reinforced by two further Tfh-targeting models in the same paper (Sh2d1a⁻/⁻ and CD4^Cre^Bcl6^fl/fl^) giving the same direction.

Contradictions & Debates

None documented in current wiki sources.

Related Pages

Bcl-6, CXCR5, Germinal Center, T-bet, TNF-alpha

Sources

• Kaneko2020 - GC Loss and TFH Block in COVID-19
• Song2022 - Tfh Outside Germinal Centers Drive T-bet CD11c B Cells — Icos⁻/⁻ reduces T-bet⁺CD11c⁺ B cells despite increased Th1 — a Tfh-specific requirement