Cloudman S91 Syngeneic Model Service for Melanoma

The Cloudman S91 syngeneic model grows a pigmented murine melanoma in fully immunocompetent DBA/2 hosts, preserving every immune compartment that checkpoint blockade and combination immunotherapy depend on — and answering with quantified responses where the ubiquitous B16-F10 model is notoriously refractory. Alfa Cytology provides this responsive syngeneic platform as a complete preclinical service, from pigment-verified cell banking and standardized flank implantation through caliper and imaging kinetics, checkpoint-combination dosing, and full immune-profiling endpoints, giving your melanoma immunotherapy program a model that actually answers when treated.

Overview of Cloudman S91 Syngeneic Model for Melanoma

Syngeneic melanoma models fall into two families: the fast, aggressive, and largely therapy-refractory B16 line, and the older, slower Cloudman S91 — a melanoma line described by Cloudman in 1941 from a male DBA/2 mouse and carried forward as the cultured Clone M-3 — whose defining virtue is that it responds. The tumor is genuinely melanocytic: cells constitutively synthesize melanin, express the gp100, tyrosinase, and TRP-2 differentiation antigens together with MHC class I and ICAM-1, and retain MC1R expression with the classic melanocyte-stimulating-hormone responsiveness that made the line famous in melanogenesis research. In its syngeneic DBA/2 host, every arm of antitumor immunity — CD8 T cells, NK cells, dendritic cells, and the myeloid compartment — remains intact, so therapeutic effects reflect real immune engagement rather than the artifact of an immunodeficient background. Its microenvironment, while suppressive, is responsive: profiling of established tumors shows a myeloid-dominated infiltrate with approximately 34% monocytic MDSCs, M2 macrophages around 9%, and scarce T cells — the phenotype most informative for immunomodulatory combination strategies.

Fig 1. Tumor growth curves of the Cloudman S91 syngeneic model in DBA/2 mice treated with anti-PD-1 alone or in combination with the nNOS inhibitor HH044 or the iNOS inhibitor MAC-3-190, demonstrating enhanced checkpoint blockade efficacy. (Patel, Anika, et al., 2025)

The responsiveness has been quantified across three treatment classes in standardized DBA/2J configurations. Checkpoint blockade: anti-mPD-1 delivered 116% increased time to progression, anti-mPD-L1 41%, and anti-mCTLA-4 over 283% with 20% of animals rendered tumor-free — and the syngeneic S91 model appears in Genentech's FDA pharmacology review for atezolizumab, where anti-PD-L1 at 10 mg/kg intraperitoneally three times weekly suppressed established 200 mm³ tumors. Radiation: single-dose focal irradiation from 2.5 to 20 Gy produced dose-responsive growth delay with regressions at higher doses and 10% tumor-free survivors at 20 Gy — a quantified basis for radiation–immunotherapy abscopal designs. Chemotherapy: cisplatin at 5 mg/kg achieved 143% and paclitaxel at 20 mg/kg 121% increased time to progression, with no tumor-free survivors as single agents. Operationally, subcutaneous implantation of 10⁶ cells into the high axilla yields tumors doubling in about four days within standard 20–25 day study windows, with melanin providing a visible, qualitative disease-burden biomarker throughout. Engineered extensions — M3-EGFR preserving human EGFR in vivo, a HER3-high configuration benchmarking ADC formats, and the anoikis-selected S91Nadh metastatic subline — widen the questions a single syngeneic model can host.

Cell Line Information: Cloudman S91

Cloudman S91 (Cellosaurus CVCL_J817; the cultured Clone M-3, CVCL_4512; also NCTC 3960) is an adherent, epithelial-like murine melanoma with a near-tetraploid genome on array-CGH and a stable melanin-producing phenotype. Pigmentation is the line's practical signature — roughly 1–3 pg of melanin per cell across characterized subclones — and is maintained through routine culture in vitro as a verifiable quality attribute. Culture is undemanding: DMEM or DMEM/F-12 with 10% fetal bovine serum, medium refresh every two days, and gentle subculture given comparatively loose adhesion.

Our Services

Running a responsive syngeneic model well is a different discipline from running a refractory one: effects are real but modest, so every source of variance — pigment drift, implantation inconsistency, randomization timing — directly erodes sensitivity. Alfa Cytology controls those sources with per-batch pigmentation verification, standardized high-axilla implantation, caliper schedules matched to the ~4-day doubling behavior, and immune-profiling endpoints using the published CompLeukocyte-style panel, so the checkpoint, radiation, and chemotherapy responses your program generates sit on the same scale as the benchmark matrix the model is known for.

Workflow of Cloudman S91 Syngeneic Model Construction

Construction of the Cloudman S91 syngeneic model is deliberately standardized: a well-characterized inoculum, a defined implantation site, and monitoring schedules matched to the published growth kinetics. The workflow below covers the full sequence from pigment-verified preparation through immune-profile endpoints.

1. Cell Expansion and Pigmentation Verification: Cloudman S91 cells are expanded in vitro in DMEM or DMEM/F-12 + 10% FBS with medium refresh every two days; batches are confirmed mycoplasma-negative, STR-authenticated, and verified for melanin production and gp100/tyrosinase expression before release.
2. Host Selection and Acclimatization: DBA/2J mice, typically 6–8 weeks of age, are acclimatized for approximately one week under SPF conditions and health-screened before assignment to study cohorts.
3. Inoculum Preparation: Cells are detached with gentle trypsinization or EDTA (loose adhesion requires care), washed in PBS, and resuspended at the working concentration; the standard challenge is 10⁶ cells in 25–50 µL for flank implantation.
4. Flank Implantation: The inoculum is implanted subcutaneously into the high axilla — the standardized site in the published benchmark configuration — using a fine needle, with implantation day recorded as day 0.
5. Monitoring and Staging: Tumors are measured by caliper two to three times weekly, with visible pigmentation photographed as a qualitative biomarker; body weight is recorded throughout, and randomization into treatment arms occurs at a predefined volume (commonly ~100–200 mm³).
6. Treatment and Follow-Up: Test agents are dosed on study-defined schedules — checkpoint antibodies (anti-mPD-1, anti-mPD-L1, anti-mCTLA-4), focal radiation fractions delivered with image guidance, or chemotherapy — while tumor kinetics, body weight, and clinical condition are tracked to study end, historically at 20–25 days in untreated cohorts.
7. Endpoint Analysis and Reporting: At termination, tumors are excised, photographed for pigmentation, and weighed; lungs and draining lymph nodes are screened for metastatic spread; tissues are processed for H&E and tyrosinase/gp100 immunohistochemistry, tumor-infiltrating immune populations are profiled by flow cytometry using the published panel (CD45, CD11b, MDSC, M1/M2 macrophage, NK, and T-cell subsets), and tumor-growth, time-to-progression, tumor-free-survivor, and survival data are compiled into the final report.

Fig 2. Cloudman S91 Syngeneic Model construction workflow.

Case Study-Cloudman S91 Syngeneic Model Development

Cloudman S91 is the benchmark responsive syngeneic melanoma in industry pharmacology programs, and the platform has carried a broad range of Alfa Cytology client studies — checkpoint inhibitor combinations evaluated against the published PD-1/CTLA-4 response matrix, radiation–immunotherapy pairings with image-guided focal delivery, and engineered-variant programs exploiting M3-EGFR or HER3-high configurations for targeted agent formats. Across these studies, readouts have combined caliper kinetics in vivo, pigmentation documentation, time-to-progression and tumor-free-survivor analysis, and immune-profile flow cytometry matched to the model's myeloid-dominated microenvironment. Anonymized case summaries and representative deliverable formats are available on request.

Why Choose Alfa Cytology?

The value of a responsive syngeneic model is measured by the credibility of its benchmark comparisons — and credibility comes from operational control. Alfa Cytology has organized its S91 service around the controls this model rewards.

• A melanoma that answers: quantified checkpoint, radiation, and chemotherapy response benchmarks where B16-F10 is refractory.
• Full immune competence in DBA/2 hosts — checkpoint and combination effects reflect genuine antitumor immunity.
• Pigmentation verified per batch, protecting the differentiation-antigen biology that imaging and IHC readouts assume.
• A characterized immune landscape (M-MDSC-dominated, T-cell-poor) informing combination design and flow-cytometry endpoints.
• A published, multi-class benchmark matrix available as in-study reference arms across PD-1, PD-L1, CTLA-4, radiation, cisplatin, and paclitaxel.
• Regulatory pedigree: the model appears in Genentech's atezolizumab FDA pharmacology submission.
• Engineered variant compatibility, including human EGFR preserved in vivo and a HER3-high configuration for ADC benchmarking.

Contact Us

If your melanoma program has outgrown the non-responder, Cloudman S91 offers a syngeneic system with quantified checkpoint, radiation, and chemotherapy benchmarks already on the books — and Alfa Cytology can have it running for your program to specification. Reach out to us to discuss benchmark-arm designs, radiation combination formats, and immune-profiling endpoints, and receive a customized study quotation today.

For research use only. Not intended for any clinical use.