Restricted changes in major surface protein-2 (msp2) transcription after prolonged in vitro passage of Anaplasma phagocytophilum

BACKGROUND: Anaplasma phagocytophilum strains often vary in Msp2 expression, a situation assumed to be related to immune evasion. However, Msp2 is also an adhesin, and little is known about the role of endogenous msp2 transcriptional changes in the absence of immune selection. Thus, Msp2 profiles and msp2 transcripts of low passage A. phagocytophilum Webster strain, initially comprised of a single abundant msp2 transcript, were re-examined after ≥ 20 in vitro passages without immune selection. RESULTS: Using an Msp2 monoclonal antibody, immunoblots revealed an unchanged dominant band and several weak bands that appeared with passage. Similarly, msp2 transcript diversity changed, with a decrease in the initially abundant low passage transcript and appearance of a newly abundant and several minor msp2 transcripts with high passage. BLASTN search of the A. phagocytophilum HZ strain genome revealed ≥ 52 msp2 paralogs. CONCLUSIONS: Msp2 expression and msp2 transcription modulate even without immune selective pressures. However, the limited diversity of msp2 transcripts in the absence of immune pressure suggests selection for Msp2 by specific functions beyond that of immune evasion, in spite of a large genomic reservoir for Msp2 diversity.