IL-10 Receptor Deficiency Aggravates Exhaustion of CD8⁺ T-Cells and Impedes Their Control of Chronic Lymphocytic Leukemia

Chronic lymphocytic leukemia (CLL) is associated with substantial alteration in T-cell composition and function. However, the role of T-cells in CLL remains largely controversial. While T-cells can induce pro-survival signals in leukemic cells, there are several indications for existing, yet defective, anti-tumor immune responses in CLL patients. Here, we utilized the Eµ-TCL1 (TCL1) mouse model of CLL to delineate whether anti-tumoral immune responses exist in CLL, and characterize the phenotype and function of these potential anti-tumoral immune populations.

First, we adoptively transferred TCL1 leukemic cells (TCL1 AT) into Rag2^-/- mice, which lack B- and T-cells, and observed that these mice had significantly shorter survival compared to wild type (WT) recipients. Antibody-mediated depletion of CD8⁺, but not CD4⁺, T-cells (Figure 1A), or neutralization of IFNγ after TCL1 AT resulted in a substantial increase in CLL cell numbers in blood, bone marrow and secondary lymphoid organs, indicating that CD8⁺ T-cells play a crucial role in suppressing CLL progression in an IFNγ-dependent manner.

Thus, we further studied the composition of CD8⁺ T-cells after TCL1 AT, and thereby identified a population of CD127^low CD44^int-hi effector cells that progressively increase along with disease. These cells presented the highest levels of activation markers (CD69, CD137 and GITR), degranulation capacity and Granzyme B (GzmB) production. Moreover, this population was composed of oligoclonal T-cells, as shown by T-cell receptor sequencing, indicating an enrichment of tumor-reactive T-cells. Of interest, expression of the inhibitory receptor, PD-1, was strictly confined to this effector CD8⁺ population, and we detected two distinct PD-1⁺ subsets, PD-1^int and PD-1^hi, that exhibited considerable phenotypical and functional differences. PD-1^hi cells showed higher levels of activation, yet with increased expression of exhaustion markers (Lag3 and CD244). In contrast, PD-1^int cells possessed superior functional capacity, as evident by higher cytokine (IFNγ, IL-2 and TNFα) release, stronger degranulation capacity and increased GzmB production. Gene expression profiling (GEP) revealed that PD-1^int cells resemble effector T-cells in acute infections, while the PD-1^hi subset is more similar to exhausted T-cells in chronic infections. Accordingly, PD-1^hi cells expressed high levels of transcription factors mediating terminal differentiation, such as Batf and Blimp1, while PD-1^int cells expressed memory transcription factors, like Tcf7.

As GEP data showed that PD-1^int cells expressed higher levels of Il10rb, we studied the role of the immunoregulatory cytokine IL-10 in controlling the balance between PD-1^int and PD-1^hi subsets. Using IL-10-GFP (tiger) mice, we observed IL-10 production by multiple cell types upon TCL1 AT. To assess the role of IL-10 in CLL, we treated CLL-bearing mice with IL-10R-blocking antibodies, and unexpectedly detected higher CLL load in all analyzed organs in anti-IL-10R-treated mice. Although effector CD8⁺ T-cells exhibited higher activation levels after IL-10R blockade, they were severely skewed towards PD-1^hi cells with an almost complete loss of the PD-1^int subset. This was accompanied by higher expression of inhibitory receptors and a substantial decrease in GzmB release and cytokine production. Moreover, GEP revealed a dramatic decrease in cell cycle and proliferation genes in PD-1^hi cells following IL-10R blockade, which was further confirmed by Ki67 staining. To examine whether these effects are T-cell intrinsic, we transferred CD8⁺ T-cells from WT or Il10rb^-/- mice into Rag2^-/- mice followed by TCL1 AT. While WT T-cells substantially decreased the rate of CLL progression, Il10rb^-/- cells were significantly inferior in controlling CLL in these mice (Figure 1B). Moreover, Il10rb^-/- CD8⁺ T-cells were mostly PD-1^hi cells with an increase in exhaustion markers and substantial drop in cytokine and GzmB production.

Collectively, this study describes a novel role of IL-10 in maintaining immune responses in CLL by protecting anti-tumoral effector CD8⁺ T-cells from activation-induced exhaustion. Furthermore, these data suggest IL-10 as a therapeutic target in CLL, potentially in combination with immune checkpoint inhibitors.

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