Tumor Implantation Site and Acquired Resistance to Anti-PD-1

Date:

Authors:

Morgane Denis Doriane Mathe Manon Micoud Pierre-Antoine Choffour Chloé Grasselly Eva-Laure Matera Charles Dumontet

Download pdf:

Impact of Tumor Implantation Site on Anti-PD-1 Response and Acquired Resistance in Mouse Models

This study investigates whether the anatomical site of tumor implantation in mice influences the tumor immune microenvironment (TIME), the response to anti-PD-1 immune checkpoint therapy, and the characteristics of acquired resistance. The authors compare MC38 colorectal tumors implanted subcutaneously (SC) versus orthotopically (cecum). :contentReference[oaicite:1]{index=1}

Study Design

  • MC38 colorectal tumors were established in C57Bl/6 mice either by SC injection or by orthotopic grafting onto the cecum.
  • Anti-PD-1 (clone RMP1-14) was administered weekly (12.5 mg/kg, intraperitoneal), initiated at ~150 mm3 for SC tumors, or 48 hours post-surgery for orthotopic tumors.
  • Acquired resistance was generated by serial reimplantation of responding tumors under continued anti-PD-1 selection pressure (≥ 5 passages for SC; ≥ 7 passages for orthotopic models).
  • TIME was characterized by flow cytometry at baseline and after treatment.

:contentReference[oaicite:2]{index=2}

Key Findings: Sensitivity to Anti-PD-1

Both SC and orthotopic MC38 models demonstrated sensitivity to anti-PD-1 therapy. Orthotopic tumors grew more aggressively, which led to earlier treatment initiation in that setting. :contentReference[oaicite:3]{index=3}

Key Findings: Acquired Resistance Can Be Induced in Both Models

Following repeated passages under anti-PD-1 treatment pressure, both SC and orthotopic MC38 tumors acquired a resistant phenotype, defined by similar tumor progression in treated versus untreated animals. :contentReference[oaicite:4]{index=4}

Baseline TIME Differs Strongly by Implantation Site

At baseline, the immune composition of tumors was markedly different depending on implantation site. In SC sensitive tumors, a large proportion of CD11b+ cells was detected along with B and T lymphocytes and NK cells. In contrast, orthotopic sensitive tumors showed a high proportion of B cells and PMN-MDSCs, with lower representation of other myeloid cells, T cells, and NK cells. :contentReference[oaicite:5]{index=5}

After Treatment, TIME Tends to Converge Between Sites

After anti-PD-1 therapy, immune infiltrates tended to become more similar across implantation sites. In both sensitive SC and orthotopic tumors, treatment was associated with decreased B cells and increased F4/80+CD206+MHC-II+ cells. In orthotopic sensitive tumors, therapy also significantly reduced PMN-MDSC infiltration. :contentReference[oaicite:6]{index=6}

Resistance-Associated Immune Landscape Depends on Site

In resistant tumors, baseline TIME remained strongly site-dependent. The SC resistant model showed higher proportions of TAM M2-like cells. The orthotopic resistant model showed absence of T-cell infiltration at baseline, strong pDC infiltration, and strong PMN-MDSC and B-cell infiltration. Under ongoing therapy, M2-like cells decreased and CD11b+ B cells increased in both models; PMN-MDSC dynamics differed between sites but converged toward similar final proportions. :contentReference[oaicite:7]{index=7}

Conclusion

The authors conclude that tumor implantation site is a major confounding factor in preclinical immunotherapy studies: baseline immune microenvironments differ substantially between SC and orthotopic models, and resistance-associated immune features also vary by site. These results emphasize the importance of considering implantation site when modeling anti-PD-1 therapy and acquired resistance in mice. :contentReference[oaicite:8]{index=8}

Download PDF