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Core Evidence: EC MHC, Not Perfusion, Drives Rejection

The authors grafted 3D-printed human skin (single-donor keratinocytes, fibroblasts, pericytes, and ECs) onto MISTRG6 mice engrafted neonatally with human HSCs, then boosted adults with autologous PBMCs to supply alloreactive effector memory T cells plus functional human myeloid APCs — an innovation targeting the myeloid and memory deficits of standard humanized models MISTRG6 skin graft rejection preprint. Both WT-EC and KO-EC grafts vascularized equally (human CD31+ vessels: 36±9.4, 41.8±16.7, and 49.3±5.1 across groups; n=4,4,3), isolating antigen presentation from perfusion. Rejection occurred only when ECs expressed MHC I/II: histologic injury scores were 1.79±0.70 (WT-EC, n=12) vs 0.20±0.27 (KO-EC, n=5; p=0.0006) and 0.05±0.16 (no PBMC, n=9; p<0.0001) Histologic rejection scores across graft groups.

Rejecting WT-EC grafts were infiltrated by CD45RO+ memory T cells (221±131 vs 0 in controls, n=8; KO-EC 34±34, n=3) and effector CD8+ T cells (130±118 vs 2.4±2.3; KO-EC 12±10), with granzyme B expression and E-selectin upregulation on graft ECs Graft infiltrating lymphocyte quantification.

Systemic Response and Mechanistic Interpretation

TCRβ sequencing revealed shared dominant clones between skin and spleen in 2 of 3 donor-matched pairs (distinct across animals), which the authors interpret as a systemic alloresponse possibly driven by semidirect presentation via shed EC microparticles bearing intact MHC TCR clonal sharing between graft and spleen. The EC-as-APC concept builds on decades of work showing endothelial class II induction is immunologically functional Classic endothelial class II antigen presentation study.

Critical Appraisal

• Small, unbalanced n: KO-EC groups (n=3–5) vs WT-EC (n=8–12); t-tests on small groups without nonparametric correction risk inflated significance.
• Necessity vs sufficiency framing: "Sufficient" is inferred from WT-EC rejection, but keratinocyte/fibroblast MHC was intact in both arms — the design shows EC MHC is necessary and that EC-bearing grafts can be rejected, not strictly that EC MHC alone suffices.
• Unresolved variables: Whether class I alone or both classes are required, alloantibody contributions, and long-term fate (experiment stopped at 3–4 weeks because WT recipients became fragile) are acknowledged by the authors.
• Model artifacts: IL-6 knock-in, 3 skin donors, and a single HSC/PBMC donor per experiment may limit generalizability; the semidirect microparticle mechanism is interpretive, not directly demonstrated.

Falsification test: rejection of KO-EC grafts over longer follow-up via indirect presentation, or rejection of WT-EC grafts in myeloid-deficient conditions, would overturn the necessity/sufficiency claim. Confidence: moderate — internally consistent, quantitatively reported, but preclinical with limited replication power.

 BGPT Paper Review

Study Novelty

First use of MISTRG6 mice with neonatal HSC + adult autologous PBMC boost to study rejection of 3D-printed vascularized human skin; the necessary/sufficient EC-MHC conclusion in a myeloid-competent humanized system is new, though the EC-as-APC concept itself is longstanding.

Scientific Quality

Rigorous design with blinded histology, TCR sequencing, and multiple controls, but small and unbalanced group sizes (KO-EC n=3–5), unpaired t-tests only, incomplete control coverage across experiments, and 'sufficient' wording overreaches because non-EC MHC was intact.

Study Generality

Directly relevant to vascularized tissue engineering broadly (skin as tractable prototype), but findings are skin-specific and not yet tested in solid-organ or alloantibody settings.

Study Usefulness

Identifies ECs as the immunogenic gatekeeper in vascularized engineered grafts, directly informing EC-specific immune-evasion strategies (e.g., MHC editing) for off-the-shelf skin substitutes.

Study Reproducibility

Methods are detailed (bioprinter, CRISPR protocol refs, flow panels, scoring rubric) and data promised on Dryad, but MISTRG6 availability is limiting, group compositions varied between experiments, and no accession numbers were given.

Explanatory Depth

Provides mechanistic dissection (direct vs semidirect pathways, CTL effectors, systemic clonal sharing), but the semidirect microparticle mechanism remains interpretive rather than demonstrated.