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Fleur S. Peters et al.
future research direction for the epigenetic regulation of CLL-induced T-cell dysfunction. The epigenetic mecha- nisms discussed here are DNA methylation, histone mod- ification and chromatin structure; transcriptional regula- tion by non-coding RNA is outside the scope of this paper. We outline the features of CLL-induced T-cell dysfunc- tion, the molecular mechanisms of functional and dys- functional T cells and the epigenetic regulation of T-cell differentiation. Finally, we argue that the analysis of malignant B cells needs to be complemented with similar approaches in T cells of CLL patients in the future to fully understand how these immune cells affect each other in CLL. With this we intend to provide a framework for future studies into CLL-induced T-cell dysfunction.
Chronic lymphocytic leukemia-induced T-cell dysfunction
In a healthy T-cell response, presentation of antigen combined with the appropriate cytokine costimulation induces an expansion of effector T cells, followed by a contraction phase under antigen clearance. However, in the case of prolonged exposure to antigen, such as in chronic viral infection and cancer, T cells upregulate inhibitory receptors and gradually become dysfunction- al.26,27 Due to interactions with CLL cells, T cells acquire immune dysfunction that worsens during disease devel- opment.28,29 It is tempting to suggest that T-cell dysfunc- tion contributes to an increased risk for infection and a diminished capacity to control CLL progression12,30 although formal proof is currently lacking.
CD4+ T cells have been reported to have at least two tumor-promoting roles in CLL.31,32 Firstly, Th cells provide survival signals to CLL cells in the LN through contact- dependent interaction, e.g., CD40/CD40L, and by secret- ing soluble factors such as IL-4, IFNγ and IL-21 (Figure 1). These signals promote both the upregulation of anti-apop- totic proteins such as B-cell lymphoma-2 (BCL-2) family members8,33 and tumor proliferation.9 Secondly, regulatory T cells (Tregs) mediate immune tolerance and these cells are found in higher frequency in CLL patients, leading to a dampened immune response.28,34 Whether there is Th1 or Th2 skewing in CLL is still under debate as studies have resulted in contradictory findings.31,35,36
Cytotoxic T cells in CLL patients show increased expression of inhibitory receptors. In addition, functional defects such as reduced synapse formation, proliferative capacity and cytotoxicity are reported.37 These features are indicative of T-cell dysfunction, though cytokine produc- tion appears to be intact or even enhanced.24
Both CD4+ and CD8+ T cells from CLL patients display an altered subset distribution, with a reduced proportion of naive T cells and skewing towards differentiated effec- tor cells.38,39 Whilst this may partially explain the reduced immune response with autologous T-cell-based therapies in CLL,12,25 it remains unknown whether this is the result of prolonged exposure to CLL antigens as the prevalence of CLL antigen-specific T cells is unknown.
Several studies have demonstrated higher expression inhibitory receptors such as PD-1, CD160 and CD244 on CD4+ and CD8+ cells on CLL T cells.24,36,40,41 Persistent PD-1 expression is a hallmark characteristic of T-cell exhaustion in both tumor and chronic infection models42 and is often co-expressed with other inhibitory markers such as T-cell-
immunoglobulin and mucin-domain containing-3 (TIM-3) and cytotoxic T-lymphocyte-associated protein-4 (CTLA4) on CD8+ T cells, and CTLA4 and T-cell immunoreceptor with Ig and ITIM domains (TIGIT) on CD4+ cells.32,36 All of these surface proteins exert an inhibitory effect on T cells when engaged with their spe- cific ligand, and as such represent an opportunity for ICB therapy.
The presence of inhibitory markers indicates dimin- ished T-cell function which is substantiated by direct func- tional studies. Research shows that CLL T cells have impaired synapse formation.35,43 Proliferation is also affect- ed as Riches et al.24 showed a reduced proliferative index of CD8+ T cells after a αCD3/αCD28 stimulation when comparing CLL patients to age-matched healthy donors. Additionally, even though these CD8+ T cells retained the capacity to degranulate in a killing assay, killing of target cells was defective in CLL T cells.24 In contrast, others described reduced degranulation in CLL CD8+ T cells com- pared to healthy donors.25,37 These contradictory findings could reflect a difference in experimental design or high- light different stages of dysfunction between patients.
CLL cells and Tregs produce the anti-inflammatory cytokines IL-10 and transforming growth factor-b (TGF-b) and thereby create an immunosuppressive environment.44 Since Tregs are present at higher frequency in CLL, it is likely that these cytokines play a role in the T-cell dysfunc- tion. Additionally, Jadidi-Niaragh et al.45 demonstrated that the expansion of Tregs coincided with a decrease in IL-17 producing cells during CLL progression. IFNγ, TNF and granzyme B production was not reduced in CLL CD8+ T cells compared to age-matched healthy donors.46,47 Riches et al.24 even reported enhanced expression of IFNγ and TNF. These findings seem to reflect a heightened pro- inflammatory state of CD8+ T cells in CLL .
The majority of aforementioned studies describing CLL- induced T-cell dysfunction were derived from the analysis of PB rather than the LN, that more accurately reflects the physiological CLL TME. T cells from the PB and LN com- partments exhibit phenotypic differences, with LN- derived T cells exhibiting signatures that reflect recent TME interactions. The dysfunctional T-cell phenotype is more severe in the LN,10,28 suggesting the analysis of PB- derived T cells is compromised. In addition, bulk analyzes lead to a mixed phenotype. Analyzing paired LN and PB samples with single-cell approaches such as high-dimen- sional flow cytometry or single-cell transcriptional and epigenetic sequencing technologies would reveal mecha- nisms of dysfunction that are not detectable in bulk PB samples.
In conclusion, T-cell abnormalities in CLL patients include altered cytokine secretion profiles, an exhausted phenotype and compromised cytotoxicity which might be more prominent in the CLL TME. In-depth characteri- zation of both LN and PB compartments will hopefully enhance our understanding of the interactions between CLL cells and T cells that induce T-cell dysfunction.
Transcriptional control of T-cell function and dysfunction
Cancer-induced T-cell exhaustion and dysfunction is mainly caused by the interactions with malignant cells within the TME,26 through ongoing engagement of
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haematologica | 2021; 106(5)