Mechanisms of Therapeutic Response to Tipifarnib in a Mouse Model of Angioimmunoblastic T-Cell Lymphoma

Angioimmunoblastic T-cell lymphomas (AITLs) are aggressive non-Hodgkin lymphomas arising from malignant transformation of follicular helper T-cells (Tfh) and are associated with limited response to intensified chemotherapy and poor prognosis. The genomic landscape of AITL is characterized by loss of function mutations in epigenetic regulators, including TET2, and the highly prevalent RHOA G17V mutation identified in almost 70% of AITL cases (Palomero et al., 2014). Using a combination of loss of TET2 and expression of RHOA G17V in CD4⁺ T-cells, our group developed a new mouse model of AITL, which accurately recapitulates the features of the human disease (Cortes et al., 2018). Our model constitutes a relevant platform for studying AITL pathogenesis and developing experimental therapeutic approaches for the treatment of this disease.

Tipifarnib is a farnesyltransferase inhibitor that has been linked to reduced cell proliferation and increased apoptosis in a variety of solid tumors and leukemias (Alsina et al., 2004; Kirschbaum et al., 2011). Interestingly, a phase II study of tipifarnib in patients with relapsed/refractory lymphoma indicated that, while tipifarnib had a modest anti-lymphoma activity within the whole cohort, it elicited an overall response rate of 31% within the Peripheral T-cell Lymphoma cases, particularly AITL (Witzig et al, 2011). However, the molecular bases for the therapeutic activity of tipifarnib in AITL are still unknown.

To understand the role and the mechanisms of action of tipifarnib in AITL, we have used our TET2^-/- RHOA G17V mouse AITL lymphoma model. Using this model, we first identified that treatment in vitro with increasing doses of tipifarnib showed limited effect on the viability of AITL lymphoma cells; however, when mouse lymphoma cells were co-cultured in the presence of splenocytes as supporting cells, tipifarnib induced a strong decrease in cell viability and proliferation, suggesting that tipifarnib might be exerting its therapeutic effects not only on the tumor cells, but also indirectly by regulating the tumor microenvironment. Indeed, tipifarnib treatment in a lymphoma model in vivo led to significantly decreased tumor load and substantial reduction of lymphoma cell infiltration in solid organs, demonstrating a consistent and strong anti-tumor effect of tipifarnib in AITL in vivo. Mechanistic analysis of signaling pathways regulated by tipifarnib indicated that tipifarnib induced downregulation of surface expression of the CXCR4 receptor in tumor cells and increased the circulating levels of its ligand, CXCL12, resulting in an effective increase of the CXCL12/CXCR4 ratio. In addition, our data revealed a significant tipifarnib-induced decrease in the levels of IFN-G, TNF-A, IL-10 and IL-17 cytokines.

We also performed gene expression profiling in sorted AITL lymphoma cells and sorted stroma splenocytes from mice that had been treated with tipifarnib or vehicle control. Interestingly, most of the genes specifically affected by tipifarnib were expressed in stromal cells, where we detected an upregulation of Fas-L and, more notably, of multiple genes associated with NK cell function and differentiation (CD226, CD244a, Klre1, Xcl1 and Eomes). CYBERSORT analysis of hematopoietic cell populations indicated that treatment with tipifarnib in our mouse lymphoma model is associated with enrichment in NK signatures and a potential decrease of monocytes and neutrophils within the tumor microenvironment.

In summary, we have demonstrated that tipifarnib has a strong anti-lymphoma effect on a mouse model of AITL and that this effect might be mediated by the recruitment and/or activation of different hematopoietic cell populations in the tumor microenvironment that can alter the survival and proliferation of lymphoma cells.