Sequential analysis of A. phagocytophilum msp2 transcription in murine and equine models of human granulocytic anaplasmosis

Department of Molecular and Comparative Pathobiology and Department of Pathology, The Johns Hopkins University School of Medicine, Baltimore, MD; Department of Epidemiology and Medicine, School of Veterinary Medicine, University of California, Davis

^* To whom correspondence should be addressed. Email: dscorpio{at}jhmi.edu.

	Abstract

Anaplasma phagocytophilum causes human granulocytic anaplasmosis (HGA) by inducing immunopathologic responses. Its Msp2 immunodominant protein is encoded by a family of >100 paralogs. Msp2 (msp2) expression modulates in the absence of immune pressure, and prolonged in vitro passage modulates in vivo virulence. Because programmed MSP2 expression occurs in A. marginale, we hypothesized a similar event in A. phagocytophilum in vivo, with specific Msp2 expression triggering immunopathologic injury or clinical manifestations of disease. We examined msp2 transcripts in 11 B6 mice and 6 horses inoculated with low (lpAp) or high (hpAp) passage A. phagocytophilum Webster strain. Blood was sequentially obtained through 3 weeks post-infection for msp2 RT-PCR. Horses were additionally assessed for clinical manifestations, seroconversion, CBC, blood chemistry, and cytokine gene transcription. In both species, there was no consistent emergence of msp2 transcripts, and all 22 msp2 variants were detected in both passage groups. Clinical severity was much higher for hpAp than lpAp horses, preceded by higher levels of blood IFN transcription on d7. Antibody was first detected on d7 and all horses seroconverted by d22, with a trend toward lower antibody titer in lpAp animals. Leukocyte and platelet counts were similar between experimental groups except on d13, where lpAp animals had more profound thrombocytopenia. These findings corroborate studies in mice where msp2 diversity does not explain differences in hepatic histopathology, but differ from the paradigm of lpAp causing more significant clinical illness. Alteration in transcription of msp2 has no bearing on clinical disease in horses, suggesting the existence of a separate proinflammatory component differentially expressed with changing in vitro passage.