PECAM1 and myeloid sarcoma: Upregulated vascular PECAM-1 expression in initial (pre-phagocytic) white matter as well as active cortical gray matter MS lesions thus might represent a compensatory mechanism to repair BBB characteristics and to prevent uncontrolled immune cell infiltration via the transcellular pathway, which is seemingly facilitated by inflammatory cytokine (e.g., IL-1β)-mediated ICAM-1 upregulation and ICAM-1 signaling in brain endothelial cells.