The role of cellular immunity in various forms of glomerulonephritis (GN), including crescentic GN, is increasingly recognized. Several studies have demonstrated involvement of T helper cells and myeloid cells in GN, but the contribution of cytotoxic CD8+ T cells is unclear. Previous work by Heymann and colleagues demonstrated in vitro killing of podocytes by podocyte-specific antigen-directed CD8+ T cells but could not demonstrate direct interaction of these T cells with podocytes in vivo. The development of new tools has now enabled researchers to show that cytotoxic CD8+ T cells can attack podocytes in vivo, but only under circumstances where breaches in the Bowman’s capsule enable T cells to access the glomerulus. “Our major finding is that podocytes are not normally accessible to cytotoxic CD8+ T cells, but after induction of nephrotoxic serum nephritis, breaches in the Bowman’s capsule allow podocyte-targeted T cells to directly attack and destroy podocytes, resulting in rapidly progressive and destructive GN,” explains Detlef Schlondorff.

Credit: Lara Crow/Springer Nature Limited

To assess whether CD8+ T cells were able to access podocytes, Schlondorff and colleagues took advantage of two genetic mouse models. In one of these models, CD8+ T cells had been engineered to express a T cell receptor that recognizes enhanced green fluorescent protein (EGFP). The researchers injected T cells from these mice — termed just EGFP death inducing (Jedi) mice — or from control mice into transgenic mice that specifically expressed EGFP in podocytes (pod-EGFP mice). “Combining the Jedi CD8+ T cells with transgenic pod-EGFP mice allowed us to examine the interaction of Jedi T cells with EGFP-expressing podocytes under control conditions and under conditions of experimental nephrotoxic serum nephritis,” says Schlondorff.

Injection of control CD8+ T cells or Jedi CD8+ T cells into pod-EGFP mice did not induce kidney damage or proteinuria. Co-injection of EGFP-expressing recombinant lentivirus resulted in the transduction of some splenocytes with EGFP, which were subsequently eliminated by Jedi CD8+ T cells, demonstrating that the Jedi T cells were activated and suggesting that the inability of Jedi CD8+ T cells to eliminate EGFP-expressing podocytes might be due to the presence of a protective niche. To test this hypothesis, the researchers induced a mild form of nephrotoxic serum nephritis in pod-EGFP mice before the injection of Jedi or control T cells. These mice rapidly developed proteinuria, which subsequently declined in mice that received control T cells. Injection of Jedi T cells, however, exacerbated glomerular dysfunction. Histological analysis showed that kidneys of pod-EGFP mice injected with either nephrotoxic serum only or with nephrotoxic serum plus control T cells had mild to moderate crescent GN; by contrast, kidneys of mice injected with nephrotoxic serum plus Jedi T cells had severe lesions with more crescents, defects in the Bowman’s capsule, podocyte apoptosis, as well as periglomerular and interstitial infiltrates. Glomeruli with crescents — particularly those with ruptures or loss of the Bowman’s capsule — exhibited the greatest loss of podocytes, with direct interactions between Jedi T cells and EGFP+ podocytes evidenced by 3D fluorescent deconvolution microscopy. Of note, infiltration of Jedi CD8+ T cells was only observed in glomeruli with crescents and breaches in the Bowman’s capsule; in glomeruli with an intact Bowman’s capsule, Jedi CD8+ T cells accumulated externally and did not interact with podocytes. “We demonstrated similar findings in biopsy samples from patients with rapidly progressive GN, where glomerular CD8+ T cell infiltration also correlated with breaches in Bowman’s capsule,” comments Schlondorff.

The researchers say their work strengthens the role of CD8+ T cells in the progression of rapidly progressive GN and supports the use of therapeutic interventions against CD8+ T cells in crescentic GN. “Our demonstration of a novel role for the Bowman’s capsule in providing a barrier against CD8+ T cell infiltration should direct future research towards identifying the mechanisms involved in the destruction of the Bowman’s capsule in crescentic GN,” Schlondorff adds.