Specific PD-1 Deletion on Regulatory T Cells Leads to Enhanced Anti-Tumor Responses

• Published in: Blood 2023-11, Vol.142 (Supplement 1), p.2551-2551
• Main Authors: Aliazis, Konstantinos, Christofides, Anthos, Aksoylar, Halil-Ibrahim, Shah, Rushil, Pal, Rinku, Patsoukis, Nikolaos, Boussiotis, Vassiliki A
• Format: Article
• Language: English
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood-2023-174422

Programmed cell death 1 (PD-1) is a T cell inhibitory receptor and the most extensively exploited therapeutic target of checkpoint immunotherapy in cancer. Ligation of PD-1 by its ligands PD-L1 or PD-L2 leads to inhibition of CD4 + and CD8 + T cell responses. However, very little is known about the specific role of PD-1 in regulatory T (Treg) cells. Previous studies have shown that PD-1 engagement during induction of Treg polarization by TCR/CD28 signaling in the presence of IL-2 and TGF-β promoted the generation and expansion of Treg cells with potent suppressive function. It has also been shown that patients with high numbers of PD-1 + Treg cells have unfavorable responses to PD-1 checkpoint immunotherapy. Moreover, patients who developed hyperprogressive disease after PD-1 blocking immunotherapy exhibited enhanced Treg cell proliferative ability compared to patients with non-hyperprogressive disease. In the present study, we sought to understand the role of PD-1 on Treg cells in the context of cancer. We generated mice with conditional targeting of the Pdcd1 gene (encoding for PD-1) and crossed them with FoxP3Cre-YFP ( Pdcd1 f/fFoxP3Cre) resulting in specific deletion of PD-1 in Treg cells. Under steady-state conditions, the percentage of Treg cells in the spleen and lymph nodes remained comparable between Pdcd1 f/fFoxP3Cre and Pdcd1wt/wtFoxP3Cre control mice. However, PD-1-deficient Treg cells had a more activated phenotype compared to their counterparts in control mice, with higher expression of CD69, ICOS, and PD-L1. Furthermore, Treg cells in Pdcd1 f/fFoxp3Cre mice expressed increased levels of GITR, CTLA-4, ICOS, and PD-L1 compared to endogenous CD4 + T conventional cells (CD4 +Foxp3 -, Tconv) and CD8 + T cells. To investigate the implications of these immunological changes in anti-tumor responses, we used the MC38 colon adenocarcinoma syngeneic mouse model. We found a smaller tumor size in Pdcd1 f/fFoxP3Cre mice compared to their control counterparts. Treg cells from spleens and tumor-draining lymph nodes (tdLNs) showed minimal differences in their activation state, expression of checkpoint markers, and proliferation levels, as determined by direct ex vivo assessment. Similarly, CD4 + Tconv and CD8 + T cells from spleens and tdLNs showed a very low level of activation that was comparable between the two experimental groups. In the tumor site, however, Pdcd1 f/fFoxp3Cre tumor-bearing mice had a significant reduction in the fractions of CD4 + Tconv