Macrophage Accumulation and Cyst Expansion in Pkd2, Ift88, and Double Mutant Mouse Models.

Background: Kidney cyst formation occurs due to loss of cilia localized polycystin proteins (e.g., Pkd1 or Pkd2) or ciliary structure (e.g., Ift88 or Kif3a). However, cyst progression is more rapid in polycystin mutant mice compared to cilia mutant mice, and loss of cilia in the polycystin mutant background (e.g., Pkd2 and Kif3a mutation) greatly attenuates cyst development. This led to the proposal that the polycystins function to repress a cyst promoting pathway that is dependent on an intact cilium, this is referred to as the Cilia-Dependent Cyst Activating (CDCA) pathway. Renal macrophages are also involved in regulating cyst progression, but it is unknown whether this occurs through the CDCA or separate pathway.

Methods: To examine whether macrophage accumulation was regulated through a cilia-dependent pathway, we compared macrophage accumulation and cytokine expression levels in Pkd2 mutant kidneys with or without intact cilia (Ift88 mutants). To avoid the impact of cyst induced damage on macrophage accumulation, we conducted comparisons after standardizing the samples for cystic indices between the Pkd2, Ift88, and Pkd2;Ift88 double mutants.

Results: Disruption of Ift88 in Pkd2 mutants reduced cyst burden and attenuated macrophage accumulation and cytokine expression levels. However, when the mutants were standardized based on cystic indices, no significant differences in macrophage number or cytokine expression were evident between the Pkd2, Ift88, and Pkd2;Ift88 double mutants at either early or advanced stage of cyst progression and pathway analysis revealed the macrophage populations were similar between groups based on single-cell RNAseq data.

Conclusions: These data indicated that macrophage accumulation and cytokine expression did not drive cyst initiation but rather paralleled cyst expansion regardless of the genotype or rate of disease progression through a cilia independent pathway.