IL‑10 limits CLL progression by reprogramming T cell exhaustion and myeloid suppression

Despite major advances in immunotherapy, respective treatments often show only limited benefit in patients with chronic lymphocytic leukemia (CLL). Immune checkpoint blockade, which has shown durable responses in multiple malignancies, has yielded inferior efficacy in CLL. Similarly, adoptive CAR-T cell therapy, which has achieved remarkable success in other B cell malignancies, resulted in lower response rates in CLL. The lack of success of these immunotherapies is likely due to the profound T cell dysfunction and immune suppressive microenvironment, typical characteristics of this disease. The so-called “exhaustion” of T cells is marked by impaired effector capacity, sustained inhibitory receptor expression, and loss of progenitor exhausted (TPEX) populations, cell subsets which are critical for responses to PD-1 blockade. This suggests that reversing T cell exhaustion could be key to improving outcomes. Emerging evidence implicates the immunoregulatory cytokine interleukin-10 (IL-10) as a context-dependent modulator of T cell exhaustion. While traditionally viewed as an immunosuppressive mediator that limits tissue damage during chronic inflammation and suppresses antiviral immunity, IL-10 may, in cancer, enhance tumor-infiltrating CD8⁺ T cell function, prolong their survival, and delay T cell exhaustion. Using the Eµ-TCL1 adoptive transfer (TCL1-AT) mouse model of CLL, we previously identified a role for IL-10 receptor (IL-10R)–STAT3 signaling in sustaining CD8⁺ TPEX cells, preventing terminal exhaustion, with loss of signaling accelerating CLL progression (Hanna et al., Immunity 2021). As IL-10 expression is linked to better outcome in CLL patients and IL-10R loss is associated with DLBCL development early in life, we hypothesize that IL-10 improves the fitness of cytotoxic T cells thereby preventing immune escape in CLL and likely other B cell malignancies. To test the potential of IL-10 in modulating T cell responses against CLL, we generated a stabilized IL-10 fusion protein (IL-10-Fc) and demonstrated that treatment of TCL1-AT mice with IL-10-Fc slowed CLL progression, reduced exhausted effector T cells, and preserved TPEX populations. To dissect T cell-intrinsic effects, we employed an in vitro exhaustion assay by repetitive antigen-specific stimulation of T cell receptor (TCR)-transgenic murine CD8⁺ T cells with peptide-loaded target cells. IL-10 treatment maintained effector T cells in these cocultures, reduced terminal exhaustion markers TIM-3 and CD39, and promoted a polyfunctional phenotype with robust cytokine production and CD107a expression, demonstrating a direct effect of IL-10 on T cell fitness. To gain a broader view of IL-10-mediated immune modulation in the TCL1-AT model, we performed spectral flow cytometry and single-cell RNA-sequencing following short-term IL-10-Fc treatment. Both T cell and myeloid compartments displayed therapy-induced alterations, including shifts in regulatory T cells, CD8⁺ T cells, neutrophils, and