C5AR1 and Alzheimer disease: The data show that (1) adult deletion of C1q has a subtle but statistically significant impact on the microbiome in the Arctic model of AD, but not in WT mice; (2) constitutive deletion of C5aR1 significantly altered the fecal microbiome of both WT and Arc mice when C5aR1-deficient mice were housed separately from C5aR1-sufficient counterparts; this alteration was not observed in cohoused C5aR1KO and C5aR1+ mice; and (3) treatment of two AD models with the C5aR1 inhibitor PMX205 had no effect on the fecal microbiome, despite these mice being singly housed for the duration of the treatment.