Trafficking signals — specifically, a chemokine gradient within the tissue epithelial compartment — could be responsible for the movement of CD8+ T cells within the allograft. Barba and colleagues analyzed scRNA-Seq data from endothelial, interstitial, and tubular compartments and found that mRNA levels for CXCL9, CCL2, CCL5 and, most strikingly, CXCL12 were upregulated within the interstitium (11).

The authors set up an elegant killing assay to determine whether a CXCL12 gradient was sufficient to enable CD8+ T cells to bypass allogeneic endothelial cells. Under static conditions, meant to mimic the interstitial tissue environment, chemokine was present uniformly within the culture media. In contrast, dynamic chemotactic conditions, intended to mimic the transition from vasculature to interstitium, provided chemokine below the endothelial layer in a Transwell chamber. Notably, relative to static conditions, endothelial cells were less damaged by alloreactive CD8+ T cells migrating along a CXCL12 gradient under dynamic conditions. Similar results were obtained when tubular epithelial cells were cultured in the dynamic conditions, demonstrating the cell-extrinsic nature of this mechanism. In addition, the authors tested shear flow conditions to mimic CD8+ T cell trafficking through circulation in the context of inflammation. Endothelial cell damage was completely prevented in the presence of a CXCL12 gradient.

Barba et al. (11) put forward the model that a CXCL12 chemokine gradient protects the endothelium from damage in most cases of TCMR (Figure 1). A corollary of this concept posits that pathologically severe TCMR, in which the endothelium becomes damaged, would coincide with a disrupted CXCL12 gradient. In a cohort of TCMR kidney biopsies with and without endothelialitis, computer assisted histomorphometry was used to assess the CXCL12 gradient. TCMR biopsies without endothelialitis exhibited a discernible chemokine gradient, while biopsies with endothelialitis had disruption of both the distribution and intensity of the gradient.

The CXCL12 gradient enables CD8+ T cells to bypass the allograft endothelial layer and mediate rejection. Alloantigen-specific CD8+ T cells are primed in the secondary lymphoid tissue and travel via the bloodstream to the allograft. In the graft vasculature, CD8+ T cells bind to cognate peptide-HLA complexes and adhesion receptors, including integrins, selectins, and chemokine receptors, and extravasate into the graft parenchyma. Despite the presence of cognate alloantigen, the migration of alloantigen-specific CD8+ T cells leaves the donor endothelial layer largely intact. Evidence from Barba et al. (11) suggests that alloantigen-expressing endothelial cells did not have a cell-intrinsic resistance to CD8+ T cell–mediated cytotoxicity. Rather, chemokine gradients within the allograft, such as CXCL12, promoted CD8+ T cell migration through the endothelial layer and into the graft interstitium, where graft epithelial cells are susceptible to cytotoxic injury.