Regulatory T Cells Keep Pancreatic Cancer at Bay

Anticancer treatments have been revolutionized by immunotherapy, which modulates a host's immune system and overcomes mechanisms of T-cell suppression. As such, immunotherapy can result in durable tumor regression in a number of malignancies. However, the majority of pancreatic ductal adenocarcinomas (PDA) are resistant to current immune checkpoint inhibitors. Although in many cancers CD8⁺ T cells are directed toward CTL function and induce tumor clearance, in PDA their deletion or depletion does not accelerate oncogenesis (1). It is thought that multiple parallel mechanisms of immunosuppressive mechanisms weaken T-cell activation. Therefore, overcoming tumor microenvironment (TME)–induced immune suppression to improve responses to immunotherapy remains a major clinical challenge. In PDA, T regulatory (Treg) cells accumulate during the preinvasive stage of the disease (2). Historically, Treg cells are considered a homogeneously tumor-promoting population and various mechanisms of Treg cell–mediated suppression of CTL responses have been proposed, including competition for access to antigen-presenting dendritic cells (DC; ref. 3). Initial hopes for understanding the role of Treg cells in PDA came with the previously reported observation that Treg cell depletion resulted in attenuated tumor growth in an orthotopic syngeneic model of pancreatic cancer (4). Unlike CD8⁺ T cells, intratumoral Treg cells were found to engage in prolonged interactions with tumor-associated CD11b⁺ DCs and reduce their expression of costimulatory molecules. In line with these observations, depletion of Treg cells was found to turn tumor-infiltrating T cells that are dispensable to outcome in PDA into indispensable mediators of tumor protection.

Within this framework comes the study in this issue by Zhang and colleagues that addresses the critical question of the interrelationship among the various stromal cells found in PDA and Treg cells (5). Using multiplex IHC of surgical tissue samples obtained from human PDA resections, the authors demonstrated that Treg cells are present in most tumor samples and correlate with macrophage and CD8⁺ T-cell infiltration. Furthermore, they verified by immunostaining and single-cell sequencing analysis that Treg cells are present in pancreatic intraepithelial neoplasia (PanIN)–bearing Kras^+LSL/G12D;Ptf1a^+/Cre (KC) as well as iKras* and iKras*;p53* mice. To understand the role of Treg cells in the formation of PanIN, they generated Kras^+/LSL–G12D;Ptf1a^+/Cre;Foxp3^{tm3(DTR/GFP)Ayr} (KC;Foxp3^DTR) mice in which the human diphtheria toxin receptor (DTR) is expressed under the control of the Foxp3 locus. The most novel finding in this study is that the administration of diphtheria toxin to deplete Treg cells resulted in immune cell infiltration with acinar-ductal metaplasia (ADM) and dedifferentiation in KC;Foxp3^DTR pancreata. In addition, KC;Foxp3^DTR animals exhibited an increased pancreas-to-body ratio, which is a marker of tumor burden. These observations together suggest that, in the presence of oncogenic Kras, depletion of Treg cells causes a deleterious inflammation that coorchestrates PanIN formation. Although these findings are ostensibly paradoxical to previous studies, Zhang and colleagues use an autochthonous model of slowly progressing PDA. As such, the observations from this study can be compared to a limited extent to transplantation models that are much more aggressive and harbor a distinct TME.

Cancer-associated fibroblasts (CAF) represent a heterogeneous cell population. Among several other cell types, resident tissue fibroblasts are considered predecessors (6). In the pancreas, CAFs are prevalent in the stroma of PanIN lesions and can deposit extensive amount of extracellular matrix, leading to a prominent desmoplastic reaction with a dense fibrotic stroma that is characteristic of pancreatic cancer (7). To investigate whether Treg cells altered the fibroblast population in PanIN lesions, Zhang and colleagues fluorescently labeled α-smooth muscle actin (SMA), a marker for myofibroblastic CAFs (myCAF) in KC and KC;Foxp3^DTR mice. CAFs are generally considered master regulators of various tumor-supportive processes (6). Nonetheless, in the context of PDA, the exact functions of CAFs continue to be debated. As such, depletion of CAFs results in poorly differentiated tumors, enhanced metastatic spread, and reduced survival (8). The current study goes beyond these previous findings and identifies Treg cells as a novel regulator of myCAFs. To investigate the extent to which myCAFs can induce immunologic reprogramming, the team analyzed the immune cell landscape of PanIN lesions after Treg cell depletion. They treated mice with caerulein and depleted Treg cells one week before sacrifice to allow for a similar severity of PanIN lesions. This treatment protocol allows the study of immune-cell infiltration independent of the PanIN lesion load. Importantly, despite mostly unchanged T-cell numbers, depletion of Treg cells increased F4/80⁺ macrophages/DCs with an inhibitory ARG1⁺ phenotype. Subsequent analysis of the secretome revealed that the influx of immune-suppressive myeloid cells is dependent on CCR1. To test whether the influx of immunosuppressive myeloid cells upon Treg cell depletion can be abrogated by pharmacologic blockade of CCR1, the authors used the CCR1 inhibitor BX471. They found that CCR1 blockade reduced dysplasia and fibrosis despite the depletion of Treg cells. Of note, CCR1 blockade was recently shown to have a synergistic antitumoral effect when used in combination with anti–PD-L1 in a murine model of breast cancer (9).

CD4⁺ T cells play a crucial role in antitumor immunity. Typically, they recognize peptides presented by MHC II molecules expressed on the surface of antigen-presenting cells such as macrophages or DCs. Such activation results in the differentiation of naïve CD4⁺ T cells into T helper cells (Th) through activation of specific transcription factors. In pancreatic cancer, Th1-polarized CD4⁺ T cells are thought to mediate tumor protection. Conversely, Th2-polarized CD4⁺ T cells are associated with tumor-permissive anergy (1). Zhang and colleagues thus analyzed Th1 and Th2 subsets in pancreata of Treg cell–depleted mice. The team found that Treg cell–depleted KC;Foxp3^DTR had increased concentrations of Th2-associated and decreased concentrations of Th1-associated cytokines. Finally, the authors showed that depletion of CD4⁺ T cells in KC;Foxp3^DTR mice mitigated immune infiltration as well as ADM and resulted in decreased fibrosis (Fig. 1).

These findings of Zhang and colleagues add to an emerging theme that challenges the previous notion where Treg cells are seen as critical mediators of immune suppression in PDA. As such, previous work demonstrated that, despite the ostensible tolerogenic Treg phenotype induced by PDA-infiltrating DCs, CD4⁺ Treg cells have negligible tumor-promoting or immune-suppressive functions when compared with Tr1 cells (10). Not only are these new insights important to the understanding of PDA, but the mechanisms uncovered in this study could have relevance for other tumor entities with a strong fibrotic component as well. Furthermore, could these discoveries shed light on the mystery of why so far in clinic, no Treg cell–targeted therapy has been proven to be effective? The information we learn from forthcoming studies investigating this question will allow us to not only understand the biology of Treg cells better but improve upon the current clinical outcomes as well.

No potential conflicts of interest were disclosed.