Cd70 Deficiency Impairs CD4+ and CD8+ T-Cell Immune Surveillance in Bcl6-Driven Diffuse Large B-Cell Lymphomas

Anti-tumor immune responses to lymphoid malignancies are modulated by co-stimulatory and co-inhibitory pathways. Our group previously characterized the recurrent genetic alterations in primary diffuse large B-cell lymphomas (DLBCL) and identified frequent inactivating mutations or copy loss of the CD70 co-stimulatory ligand in these tumors. CD70 co-stimulation of CD27⁺ T cells induces antigen-dependent T-cell expansion and immune surveillance of normal and malignant B cells. Given the frequent combination of CD70 and BCL6 alterations in our human DLBCL series, we investigated the consequences of CD70 deficiency (Cd70^-/-) in a murine model of BCL6-driven lymphomagenesis (Bcl6^tg/+).

Cd70^-/-; Bcl6^tg/+ animals developed predominantly splenic DLBCLs with earlier onset and increased penetrance when compared to Bcl6^tg/+ mice (p<.01). Hence, to elucidate potential differences in T-cell mediated anti-tumor responses in Cd70^-/-; Bcl6^tg/+ versus Bcl6^tg/+ mice (and wild-type (WT) and Cd70^-/- animals) and to characterize the concurrent processes of B-cell transformation and immune evasion over time, we performed single-cell RNA sequencing of splenic cell suspensions from animals euthanized for symptoms and additional asymptomatic mice at pre-determined timepoints (6, 14 and 18 months). We applied unsupervised clustering and defined the major B- and T-cell types based on canonical lineage markers. Additionally, we reconstructed the splenic T-cell receptor (TCR) and B-cell receptor (BCR) sequences with the TRUST4 algorithm to assess their diversity and clonal expansion.

TRUST4 analyses revealed clonally expanded B cells that were primarily found in specific clusters. These clusters also had an increased number of aberrant cells identified with an orthogonal approach that detects copy number alterations, InferCNV. Certain of these clusters were more abundant in aged asymptomatic Bcl6^tg/+ and Cd70^-/-; Bcl6^tg/+ animals than in symptomatic mice. Aberrant cells in these clusters had a memory-like IgM⁺ phenotype (Nt5e, Zeb2, Cd38, Cd44, Itgb1) and included two discrete populations: 1) previously described age-associated/autoimmune-like cells (Tbx21, Cd11c, Cd11b) and 2) newly appreciated atypical memory cells. Additional clusters that were predominantly found in symptomatic animals included aberrant cells with increased expression of cycling genes and downregulation of lineage-defining markers and MHC II (p<.001). We interpret the aberrant memory-like B-cells in asymptomatic Cd70^-/-; Bcl6^tg/+ and Bcl6^tg/+ animals to be precursors of the ones detected in symptomatic mice. Importantly, malignant clonal expansion and splenomegaly occurred at earlier timepoints and to a greater extent in Cd70^-/-; Bcl6^tg/+ than Bcl6^tg/+ mice (p<.01).

To further characterize potential anti-tumor immune responses, we examined splenic CD8⁺ and CD4⁺ T-cell subsets in tumor-bearing cohorts (Bcl6^tg/+ and Cd70^-/-; Bcl6^tg/+) and control animals (WT and Cd70^-/-). In the tumor-bearing cohorts, we detected selective expansion of CD8⁺ cytotoxic T cells (CTLs) with a spectrum of activation and exhaustion markers. CD8⁺ CTL expansion was significantly greater in Bcl6^tg/+ mice than Cd70^-/-; Bcl6^tg/+ animals.

We also identified selective expansion of a major CD4⁺ CTL compartment (GzmB, Prf1, Eomes) in the tumor-bearing cohorts that persisted significantly longer in Bcl6^tg/+ than Cd70^-/-; Bcl6^tg/+ mice. In the tumor-bearing cohorts at 18 months, there was significantly greater clonal expansion of CD4⁺ CTLs, in comparison to CD8⁺ CTLs, highlighting the likely importance of this understudied cytotoxic CD4⁺ T-cell subset. In this regard, the downregulation of MHC II on the malignant B cells of symptomatic mice likely favored CD4⁺ CTL immune escape and accelerated the development of overt lymphoma.

Taken together, our data indicate that genetic perturbation of CD70/CD27 co-stimulation limits the development of an effective T-cell mediated immune response to Bcl6^tg/+-driven DLBCL and highlights the unanticipated role of CD4⁺ CTLs and MHC II down-regulation in the attempted development and subsequent loss of an anti-tumor immune response. Additionally, we identified previously unappreciated precursor populations of murine DLBCL that warrant further investigation in human tumors.