Genetic Perturbation of CD70/CD27 Co-Stimulation Promotes the Development of Bcl6-Driven Diffuse Large B-Cell Lymphoma

Multiple co-stimulatory and co-inhibitory pathways modulate T-cell dependent anti-tumor immune responses in lymphoid malignancies. We recently defined the recurrent genetic alterations and associated substructure of diffuse large B-cell lymphoma (DLBCL), including five distinct clusters (1-5), and identified potential genetic bases for immune evasion [Nature Medicine 2018; 24:679-690]. In our series, 26% of tumors had inactivating somatic mutations or copy loss of CD70 and likely disruption of CD70/CD27 co-stimulation.

CD70 and CD27 are homotrimer type II and homodimer type I transmembrane proteins and members of the TNF and TNF receptor superfamilies, respectively. CD70 is transiently expressed on certain normal B-cell and dendritic cell populations upon activation and constitutively expressed on multiple B-cell tumors. CD70 activation of CD27 ⁺ T cells promotes antigen-dependent T-cell expansion and immune surveillance of normal and malignant B cells. Patients with germline deficiencies of either CD70 or CD27 have an increased incidence of EBV-associated lymphoid malignancies, underscoring the importance of this co-stimulatory pathway.

In our series, CD70 alterations were most common in Cluster 1 DLBCLs, which also exhibited recurrent BCL6 chromosomal translocations. The co-occurrence of CD70 and BCL6 genetic alterations was noteworthy because of the established role of CD8 ⁺ T-cell dependent immune surveillance in murine models of Bcl6-driven DLBCL [Nature Medicine 2014; 20:283-290]. To assess the consequences of Cd70 deficiency and perturbed CD70/CD27 co-stimulation on Bcl6-driven lymphomagenesis, we crossed the previously described Bcl6^tg/+ and Cd70^-/- mice to generate Cd70^-/-; Bcl6^tg/+ animals. In the aging cohorts, Cd70^-/-; Bcl6^tg/+ mice were more likely than Bcl6^tg/+ animals (or the Cd70^-/- or wild-type [WT] groups) to be euthanized for symptoms before the study endpoint (18 months [mo]) (5 of 18 Cd70^-/-; Bcl6^tg/+ euthanized for symptoms prior to the first of 9 Bcl6^tg/+). Additionally, significantly greater total numbers and percentages of Cd70^-/-; Bcl6^tg/+ mice, in comparison to Bcl6^tg/+ or Cd70^-/- animals, were euthanized for symptoms (64% Cd70^-/-; Bcl6^tg/+ vs. 29% Bcl6^tg/+, p=0.005 and 64% Cd70^-/-; Bcl6^tg/+ vs. 11% Cd70^-/-, p=0.002). Almost all euthanized Cd70^-/; Bcl6^tg/+ and Bcl6^tg/+ micehad massively enlarged spleens infiltrated with histopathologically confirmed DLBCLs characterized by clonal Ig gene rearrangements. Our findings indicate that genetic perturbation of Cd70 accelerates the onset and significantly increases the incidence of Bcl6-driven DLBCL.

To characterize potential differences in the anti-tumor immune responses in Cd70^-/-; Bcl6^tg/+ and Bcl6^tg/+ mice (and Cd70^-/- and WT controls), we also harvested spleens from 6 animals in each of the aging cohorts at predetermined intervals (2, 6, 14 and 18 mo). We analyzed the composition of splenic-cell suspensions by flow cytometry and evaluated the intact splenic architecture and morphology with expert hematopathologists (VS, KW and SR). At 14 and 18 mo, spleens from WT and Cd70^-/- animals were largely normal in appearance and size. In contrast, spleens from 14 and 18 mo Bcl6^tg/+ and Cd70^-/-; Bcl6^tg/+ mice exhibited disordered architecture and abnormal pre-malignant lymphoid proliferation with expanded white pulp including morphologically and immunophenotypically aberrant B cells of small to large size and increased infiltrating T-cells. Additionally, significantly higher fractions of splenic CD8 ⁺ T cells from 14 and 18 mo Bcl6^tg/+ and Cd70^-/-; Bcl6^tg/+ animals expressed the CD69 activation marker and exhibited terminal differentiation, consistent with an ongoing anti-tumor immune response. Interestingly, Bcl6^tg/+ animals had significantly higher percentages of splenic TCRβ ⁺ T cells at the earlier time point (14 mo) with delayed-onset splenomegaly (at 18 mo), which is in marked contrast to the Cd70^-/-; Bcl6^tg/+ mice that had significantly lower percentages of splenic TCRβ ⁺ T cells at the earlier time point (14 mo) and early-onset splenomegaly (14 mo). These findings suggest that the initial T-cell mediated immune response was more effective in Bcl6^tg/+ than Cd70^-/-; Bcl6^tg/+ animals. Taken together, our data indicate that genetic perturbation of CD70/CD27 co-stimulation limits the development of an effective anti-tumor immune response in Bcl6^tg/+-driven DLBCL.