Completing the Chen-Mellman Cancer-Immunity Cycle: Sixteen positions, two cross-cancer locks, and a structural account of why anti-PD-1 fails where it fails

The Chen–Mellman cancer-immunity cycle has organised tumour immunology for thirteen years. Its seven

steps gave the field a shared language and a rational design space for checkpoint blockade. Alongside its

successes, a set of clinical observations has accumulated that the cycle as drawn cannot resolve on its own

terms: PD-L1 predicts anti-PD-1 response at AUC 0.63 across 18,792 patients; the same drug works at 40% in

melanoma and near zero in MSS colorectal; cold tumours stay cold for reasons the cycle cannot name; CAR-

T succeeds where checkpoint blockade fails through a mechanism the cycle cannot locate; five independent

laboratories converged on CXCL13 CD8 T cells as the response-predictive population without a structural

explanation; and the 2023 Mellman update extended the cycle outward into stratification axes rather than

inward into new steps — a signal from the field itself that the seven-step sequence has reached the limit of

what it can explain as drawn.

This paper proposes that the seven Chen–Mellman steps are a true description of six positions in a sixteen-

position cycle, and that the missing ten positions are precisely where the unanswered questions live. The

sixteen-position scaffold is derived independently from a universal sequence of dissipative processes (van der

Klein 2026a, b) and is used here as load-bearing infrastructure rather than as the object of argument. The cycle

traverses four regimes — Activation, Construction, Encounter, Conservation — separated by four irreversible

barriers: the Activation barrier (commitment to build a response against this antigen), the Egress barrier

(built effectors release from the lymph node into circulation), the Drain barrier (sustained killing resolves

into stable clearance), and the Suppression barrier (the maintained state generates the next antigen-cycle).

The empirical core of the paper is four results across four independent datasets:

1. Position 10 (the cytolytic synapse) is locked by a structurally derived biomarker, measured on two

independent axes within one cohort and at two measurement resolutions across cohorts. Within

Bassez breast cancer (n=29 pretreatment): CXCL13 CD8 fraction predicts anti-PD-1 response at CV

AUC 0.9746 ± 0.065 as a single feature, and the Bassez authors’ own CD8_EX label — selected without

reference to the framework — predicts at CV AUC 0.9322 ± 0.037. Two independent labelings, one po-

sition. Bulk-RNA validation: the LCAM-T module (Leader 2021) predicts anti-PD-1+chemo response

2in GSE207422 NSCLC (n=24 pretreatment) at CV AUC 0.74 ± 0.06. Five additional published cohorts

converge on the same marker in the same direction.

2. Position 8 (the licensing handshake immediately upstream of the Egress barrier) is locked across

two cancer types and confirmed by a pre-registered specificity test in a third. A Position 8 fingerprint

specified before any data was inspected — build complete, egress program absent, reserve intact, exhaus-

tion absent, CXCL13 absent — distinguishes ICI-failing MMRp tumours from ICI-responding MMRd

tumours at Fisher OR = 4.64, p = 0.0028 in the Pelka 2021 colorectal dataset (76,965 cells, 110 tumour

biosamples). The same fingerprint, applied without modification to the Bassez breast cohort, identi-

fies 12 of 20 (60%) non-responders versus 0 of 9 (0%) responders, Fisher one-sided p = 0.0024. The

framework predicts the fingerprint should be rare in cancer types where the immune cycle reaches the

Encounter phase by default; pre-registered before data inspection at <25% match in melanoma, the test

on Sade-Feldman 2018 melanoma (n=11 pretreatment anti-PD-1 monotherapy) yields 0 of 11 patients

matching the fingerprint — the strongest version of the prediction. The fingerprint’s clinical direction

also depends on the cancer type’s position on the cycle: in colorectal and breast (Construction-phase

cancer types) the built-effector phenotype is enriched in non-responders, but in melanoma (Encounter-

phase) the trend reverses, consistent with built-but-near-shipping cells being closer to where anti-PD-1

acts. One fingerprint, three cancer types, three consortiums, three scRNA platforms.

3. The single largest patient bottleneck is in the Construction phase — the lymph-node and tumour-

bed work that converts an antigen-aware system into deployment-ready effectors. A blind k-means

rebuild of 887 cancer patients across 12 tumour types from the Curated Cancer Cell Atlas places 43%

of patients in this phase, with two distinct mechanisms: myeloid substrate hijack in the tumour bed

preventing effector readiness (n=234) and Treg IL-2 sink in the lymph node interrupting clonal expansion

during priming (n=147). Neither anti-PD-1 nor any synapse-acting drug can help these patients.

4. In most cancer types, the response stalls upstream of where anti-PD-1 acts. A regime classification

of 11,373 TCGA samples across 33 cancer types finds that in only 4 of 33 cancer types does the cycle

reach the Encounter phase where anti-PD-1 acts. In the other 29, the response stalls upstream, and PD-1

release does nothing because there is no engaged-but-braked cell to release.

A fifth strand — the bidirectional ipilimumab/belatacept consistency check at the Activation barrier — confirms

the cycle’s drug assignments are bidirectional by construction. Same molecular surface (CD28–B7–CTLA-

4); same cycle position; opposite clinical goals (cancer vs kidney transplant); same two drugs working from

opposite directions on the same axis.

The implication is structural. Cancer is not primarily a failure of Encounter. Across 33 cancer types, only 4

stall at the synapse phase where anti-PD-1 acts. The remaining 29 stall earlier in the cycle (8 in the Activation

phase, 8 in the Construction phase) or in Treg over-control (13). The decade of immunotherapy effort directed

at the killing step has worked precisely in the cancers where the cycle reaches the synapse with the brake on,

and has failed precisely where the response stalls upstream. The next decade should be directed not only at

the killing step but at the build step — and in particular at the previously unnamed handshake at Position 8

immediately upstream of the Egress barrier, where built effector cells fail to receive the licensing signal that

would let them ship into Encounter at all. The drugs that act on the upstream regimes mostly already exist.

What has been missing is the map from patient to drug across the full cycle.