Adoptive hematopoietic stem cell and apd1 therapy converge in the thymus to promote antiglioblastoma immunity

Abstract Our lab previously published that adoptive hematopoietic stem cell (HSC) therapy administered in conjunction with aPD1 increases median survival in multiple models of CNS malignancies. Tumor-infiltrating lymphocytes (TIL) abundance is independently associated with better outcomes in many cancers. Impressively, approximately 25% of TILs are HSC-derived in early disease. Importantly, we show HSC therapy generates de novo TILs in a thymus-dependent fashion and offer evidence of altered central tolerance following aPD1 therapy, which may permit immune activation against a larger pool of tumor-associated antigens. Therapeutic HSCs were prepared from constitutive GFP-expressing (CD45.2+) and PD1KO (CD45.2+) mice and transferred into CD45.1+ hosts. Sublethal 5Gy total body irradiation was administered day 5 post-intracranial tumor implantation. The next day, 1E6 HSCs from a 1:1 – GFP:PD1KO cocktail were injected intravenously. Descriptive statistics are reported as mean ± SD. Transferred HSCs engraft, expand, differentiate, and migrate to form approximately 25% of the TIL compartment in only 15 days. We found that, 56.2% ± 20.3 of CD3+ TILs display a thymocyte-like CD4+CD8+ (DP) phenotype in early disease, and 43.8% ± 17.4 of these DP-TILs are HSC-derived. Interestingly, DP-TILs were virtually absent in advanced disease 29 days after treatment. At this stage, HSC-derived TILs most commonly fell within the single-positive (SP) TIL subsets, comprising 9.8% ± 5.6 of SP-CD8+ TILs. Notably, HSCPD1KO-derived cells were 4x more abundant amongst SP-CD8 TILs when compared to HSCGFP-derived cells (6.5% vs 1.6%, p < 0.05). SP-CD8 thymocytes were similarly enriched for HSCPD1KO progeny (41.1% vs 24.7%, p < 0.05) which may suggest PD-1 blockade directly promotes thymopoiesis. The privileged maturation of PD1-deficient HSCs is intriguingly reversed when GFP:PD1KO HSCs are co-transferred into non-tumor-bearing mice. Thus, we show the thymus is an active site where PD-1-blocking agents function, and that HSC + aPD1 therapy is uniquely positioned to leverage thymus-dependent anti-tumor immunity.