ICOS Costimulation Requires IL-2 and Can Be Prevented by CTLA-4 Engagement

Abstract

We investigated the relationship between ICOS, CD28, CTLA-4, and IL-2 to gain a better understanding of this family of costimulatory receptors in the immune response. Using magnetic beads coated with anti-CD3 and varying amounts of anti-ICOS and anti-CTLA-4 Abs, we show that CTLA-4 ligation blocks ICOS costimulation. In addition to inhibiting cellular proliferation, CTLA-4 engagement prevented ICOS-costimulated T cells from producing IL-4, IL-10, and IL-13. Both an indirect and direct mechanism of CTLA-4’s actions were examined. First, CTLA-4 engagement on resting cells was found to indirectly block ICOS costimulation by interferring with the signals needed to induce ICOS cell surface expression. Second, on preactivated cells that had high levels of ICOS expression, CTLA-4 ligation blocked the ICOS-mediated induction of IL-4, IL-10, and IL-13, suggesting an interference with downstream signaling pathways. The addition of IL-2 not only overcame both mechanisms, but also greatly augmented the level of cellular activation suggesting synergy between ICOS and IL-2 signaling. This cooperation between ICOS and IL-2 signaling was explored further by showing that the minimum level of IL-2 produced by ICOS costimulation was required for T cell proliferation. Finally, exogenous IL-2 was required for sustained growth of ICOS-costimulated T cells. These results indicate that stringent control of ICOS costimulation is maintained initially by CTLA-4 engagement and later by a requirement for exogenous IL-2.

Expression of ICOS on Human Melanoma-Infiltrating CD4+CD25highFoxp3+ T Regulatory Cells: Implications and Impact on Tumor-Mediated Immune Suppression

“…ICOS costimulation induces low levels of IL-2 and IL-10 that may be sufficient to accomplish such differentiation and expansion of naive CD4 ϩ CD25 Ϫ cells in the Tr1 pathway. But if the CD4 ϩ CD25 Ϫ precursors are activated via TCR and/or CD28 engagement, ICOS reinforces this interaction by supporting signaling triggered by TCR and CD28 costimulation, as previously suggested (29). This leads to differentiation and expansion of activated CD4 ϩ RC into effector, rather than regulatory, cells.…”

“…However, Taylor and colleagues (29) have recently shown that IL-10 inhibits CD4 ϩ T cell proliferation only in a defined range of TCR triggering, where T cells require costimulation. These findings suggest that CD4 ϩ T cells that differentiate into Tr1 cells upon encountering ICOS high CD4 ϩ CD25 high Treg in the tumor are suboptimally stimulated.…”

Interaction Between ICOS-B7RP1 and B7-CD28 Costimulatory Pathways in Alloimmune Responses In Vivo

“…ICOS up-regulation is inhibited in the absence of B7, but is restored following CD28 ligation (26), while CTLA4 ligation blocks ICOS costimulation by preventing cell-surface expression and downstream second signals (27). We therefore wanted to examine the effect of B7RP1 ligation on the B7-CD28 pathway by using ICOSIg therapy and assessing expression of B7.1 and B7.2 in vivo.…”

“…This is especially true for the ICOS-B7RP1, B7-CD28/CTLA4 and CD40-CD154 pathways (26,27,34). For example, T-cell ICOS up-regulation is reduced in the absence of B7.1 and B7.2, but is restored following CD28 stimulation (26), whereas CTLA4 ligation blocks ICOS costimulation by preventing cell-surface expression and downstream second signals (27). Moreover, ICOS signalling, which is essential for immunoglobulin class switching, is dependent on the CD40-CD154 pathway for this effect (34).…”

“…However, the interactions between the different pathways and their interdependence on each other are only now becoming clear. This is especially true for the ICOS-B7RP1, B7-CD28/CTLA4 and CD40-CD154 pathways (26,27,34). For example, T-cell ICOS up-regulation is reduced in the absence of B7.1 and B7.2, but is restored following CD28 stimulation (26), whereas CTLA4 ligation blocks ICOS costimulation by preventing cell-surface expression and downstream second signals (27).…”