Immunotransplant: Preventing Unintended Consequences

Adoptive T-cell transfer has now become standard therapy for many malignancies including diffuse large B-cell lymphoma (DLBCL) and is commonly used in stem-cell transplantation and chimeric antigen receptor (CAR) T-cell therapy. However, adequate lymphodepletion is required for this therapy to be successful. The reasons lymphodepletion is felt to be required are that cellular cytokine “sinks” for homeostatic cytokines including IL7 and IL15, which stimulate the infused T cells, are removed; regulatory immune cells that would inhibit effector T-cell function are suppressed; and further tumor-cell apoptosis may occur, thereby promoting activation of antigen-presenting cells (1). Of these, homeostatic activation of T cells by IL7 and IL15 is the most important, and this mechanism is uniquely different from canonical T-cell activation (2). In contrast to canonical T-cell receptor–mediated activation, which occurs via signaling through ZAP70, MAPK and ERK (3) homeostatic activation is mediated by cytokine receptors and results in JAK/STAT signaling. Homeostatic activation results in substantial expansion of mature T cells that appear antigen-experienced with a memory phenotype. These cells typically upregulate CD44 and CD122, and exhibit increased effector function including increased production of cytokines and greater cytotoxicity (2).

In this issue of Cancer Discovery, Marshall and colleagues (4) now show that homeostatic activation is coupled to a second process, namely homeostatic inhibition, which is mediated by the same cytokines (Fig. 1). IL7 and IL15 mediate, at least in part, the upregulation of PD-1 and CTLA4 on the activated T cells by inducing molecules such as NFAT that increase the transcription of PD-1 and CTLA4, as well as other immune checkpoints. PD-1 and CTLA4 ligation promotes inhibitory SHP2 signaling, thereby preventing maximal homeostatic activation of the transferred T cells. Using murine models of both lymphoma and solid tumors, they find that preemptive blocking of PD-1 and CTLA4 signaling (termed “immunotransplant” by the authors) prevents the homeostatic inhibition and promotes the antitumor T-cell response. This immunotransplant therefore effectively prevents the unintended consequence of homeostatic inhibition by the same cytokines that are meant to activate and expand the adoptively transferred T cells.

The results from this study related to homeostatic T-cell activation are highly relevant, particularly as T-cell transfer is now part of standard therapy in diseases such as DLBCL with the use of stem-cell transplantation and CAR T-cell immunotherapy. The need for lymphodepletion, as well as the increased expression of IL7 and IL15, has been found to be important in both stem-cell transplantation and CAR T-cell therapy. With CD19-specific CAR T-cell therapy, for example, the majority of patients with DLBCL have clinically measurable responses as the expanding, activated CAR T-cell population engages the malignant B-cell clone, and in at least a third of patients, these responses appear very durable (5–7). The clinical benefit, however, is directly associated with the degree of lymphodepletion (5). In clinical trials of CD19 CAR T cells in DLBCL, patients who received cyclophosphamide plus fludarabine as lymphodepleting chemotherapy had increased CAR T-cell expansion and persistence, and higher response rates (including complete remissions), than patients who received cyclophosphamide-based lymphodepletion without fludarabine (6). In fact, a higher intensity of lymphodepletion using cyclophosphamide and fludarabine was associated with higher probability of a favorable cytokine profile, which included increased levels of IL7. This favorable cytokine profile was shown to be associated with an improved progression-free survival (PFS) in patients with DLBCL in a multivariate analysis (5). Similarly, high serum IL15 levels have been associated with high peak CD19-specific CAR T-cell levels and have been associated with a greater likelihood of a clinical response (7).

Also, the findings from this study related to homeostatic inhibition are important. Upregulation of PD-1 and CTLA4 expression on activated T cells renders these cells vulnerable to inhibition due to the presence of immunosuppressive ligands in the recipient. In diseases such as DLBCL, where CAR T-cell therapy and stem-cell transplantation are commonly used, increased expression of PD-L1 and PD-L2 on both malignant cells and other cells in the tumor microenvironment has been reported. Patients with PD-L1 expression on the malignant cells were found to have an inferior overall survival when compared with patients with PD-L1–negative DLBCL (8). Furthermore, soluble PD-L1 is detectable in the plasma of patients with DLBCL and is associated with a poor prognosis. Soluble PD-L1 has also been shown to be immunologically active and able to suppress T-cell function at sites away from the tumor. Clearly, the presence of abundantly expressed immunosuppressive ligands in the peripheral blood and in the tumors of patients receiving adoptive immunotherapy highlights the fact that upregulation of PD-1 and other receptors makes activated transferred T cells highly susceptible to immunosuppression.

Whether simply blocking PD-1 and CTLA4 signaling as part of an “immunotransplant” will be sufficient to prevent homeostatic inhibition remains to be seen, and this strategy is currently being tested in a clinical trial being conducted by the authors (NCT03305445). There are data from other clinical trials to suggest that blockade of immunosuppressive signals after infusion of T cells may be a successful strategy. A previous phase II clinical trial of a mAb targeting PD-1, pidilizumab, tested the administration of this agent as post autologous stem-cell transplant consolidation for patients with DLBCL (9). The study met its prespecified efficacy endpoint, in that the PFS for this high-risk group was improved compared with the PFS that was predicted for this patient population, suggesting potential value for the strategy. However, the specificity of pidilizumab has recently been reconsidered; with some evidence suggesting that its target is in DLL1 rather than PD-1. However, the use of the PD-1 antibody pembrolizumab in patients with classic Hodgkin lymphoma after autologous stem-cell transplantation has also proved to be effective. In a phase II clinical trial, the PFS at 18 months post-transplant was 82% in this cohort, which appeared substantially greater than the expected 18-month PFS of 60% (10). The same trial tested the same strategy in a cohort of patients with DLBCL, and the results from that cohort are still awaited.

Overall, a greater understanding of both homeostatic activation and inhibition, which are coupled because of the role of IL7 and IL15 in both processes, potentially allows for prevention of T-cell inhibition to promote their activation. The proposed “immunotransplant” strategy, wherein CTLA4 and PD-1 signaling are blocked before and after lymphodepletion and subsequent administration of T cells, may avoid the unintended consequence of T-cell suppression and allow for a more effective antitumor immune response.

No potential conflicts of interest were disclosed.

S.M. Ansell acknowledges support from the Department of Defense (W81XWH1810650) and from the Leukemia & Lymphoma Society. Scientific images displayed in Fig. 1 were developed by Joseph Novak using BioRender (Toronto, Ontario, Canada).