A proposed retinal venular immune gate: a mini review of perivascular macrophages and basement-membrane organization

Leukocyte entry into the retina involves more than increased vascular permeability. This mini review synthesizes predominantly murine experimental evidence for a proposed retinal venular immune gate; the model has not been validated in the human retina. Recent studies place putative resident perivascular macrophages (pvMacs) near superficial postcapillary venules and basement-membrane low-expression regions (LERs), where matrix staining and pericyte coverage are reduced. These observations suggest that local immune signaling could interact with vascular architecture to concentrate leukocyte passage. Functional findings, however, diverge. Macrophage depletion reduces monocyte recruitment after an imposed intravitreal CCL2 challenge, whereas endotoxin-induced recruitment persists after depletion. The difference permits both stimulus-dependent macrophage participation and an alternative in which endothelial activation and permissive matrix regions govern entry without an essential macrophage organizer. We examine chemokine signaling, matrix binding, and antigen presentation as possible links, distinguishing retinal findings from mechanisms established in other tissues. In autoimmune uveitis, selective microglial MHC II studies identify an amplifying function while leaving the cellular control of earliest ocular entry unresolved. Human translation requires vessel-linked cell identification because clinical macrophage-like signals do not distinguish pvMacs from other myeloid populations. Comparing trafficking, permeability, and vascular repair after selective perturbation could reveal when perivascular macrophages influence entry and when they chiefly respond to it.