Disarming suppressor cells to improve immunotherapy

Abstract

Human tumors can use many different mechanisms to induce dysfunction in the host immune system. Accumulations of inducible regulatory T cells (iTreg, Tr1) are commonly seen in the tumor microenvironment. These Treg express CD39 and up-regulate CD73 ectonucleotidases, hydrolyze exogenous adenosine triphosphate (ATP) to AMP and adenosine and produce prostaglandin E(2) (PGE(2)). Most tumors also express CD39/CD73 and COX-2 and thus contribute to immune suppression. Pharmacologic inhibitors can be used to eliminate adenosine/PGE(2) production by Tr1 as well as the tumor or to block binding of these factors to their receptors on Teff or to selectively block cAMP synthesis in Teff. These pharmacologic blocking strategies used alone or in combination with conventional treatments or immunotherapies could disarm Tr1, at the same time restoring antitumor functions of Teff.

Fig. 1

Differential expression of surface CD39, CD73 and CD26 on lymphocyte subsets in the peripheral blood mononuclear cells obtained from normal donors (n = 15). Data are mean values ± SD. In human lymphocytes, CD73 is expressed on a small proportion of CD4⁺CD25^high Treg. The P values are for differences between marker expression on Treg and conventional CD4⁺ T cells. Data reproduced from ref. [20]

Fig. 2

Hydrolysis of exogenous ATP by human CD4⁺CD39⁺ Treg in (a) and suppression of proliferation of autologous responder T cells stimulated with anti-CD3/anti-CD28 Abs by CD4⁺CD39⁺ Treg in (b). In a, CD4⁺CD25^hiCD39⁺ Treg were incubated with exogenous ATP for 30 min. Unhydrolyzed ATP was measured. Asterisks indicate significantly lower levels of unhydrolyzed exogenous ATP in the absence of the inhibitor, ARL67156. Representative data from one of 3 experiments performed with Treg isolated from the peripheral blood of different normal donors. In b, responder T cells (R) were labeled with CFSE and co-incubated with autologous Treg (S) in the presence or absence of ARL67156. Suppression mediated by CD4⁺CD39⁺ Treg was significantly (P < 0.05) inhibited in the presence of ARl67154. The data are mean values ± SC obtained in 3 independent experiments. Data reproduced from ref. [20]

Fig. 3

Suppression of autologous effector T-cell proliferation in response to TCR-mediated signals mediated by inducible Treg (Tr1) ± ARL67154, a selective inhibitor of ATP hydrolysis or ZM 241385, an antagonist of A2a receptor. Data reproduced from ref. [23]

Fig. 4

Human Tr1 cells generated in cocultures with COX-2⁺ or COX-2- tumor cells are CD4⁺ CD39⁺COX-2⁺ and have the capability to produce adenosine or PGE₂. Tr1 cells generated in the presence of COX-2⁺ tumor cells produced more adenosine or PGE₂ than Tr1 generated with COX-2- tumor cells. The reference cultures contain conventional CD4⁺ T cells stimulated with anti-CD3/anti-CD28 Abs and proliferating in the presence of IL-2. Data reproduced from ref. [23]

Fig. 5

A schema of interactions between adenosine- and PGE2-producing Tr1 cells (CD4⁺CD39⁺CD73⁺) and effector T cells (Teff) expressing receptors for these factors. Teff that are positive for A2a receptors and EP2 receptors are highly sensitive to adenosine-mediated and PGE2-mediated suppression. Signaling via A2a and EP2 receptors converges on the intracytoplasmic adenylyl cyclase, leading to up-regulation of 3′5′-cAMP in Teff and inhibition of immune functions mediated by Teff. In a, blocking of ADO production from exogenous ATP by an ecto-ATPase, ARL 67156; in b, blocking of the binding of endogenous ADO produced by Tr1 to its receptors on Teff by, e.g, an antagonist ZM241385; or in c, blocking of PGE₂ production by Tr1 all are expected to rescue Teff from suppression. The red markers indicate blocking with pharmacologic inhibitors