Differential Immunological Responses of Adult Domestic and Bighorn Sheep to Inoculation with Mycoplasma ovipneumoniae Type Strain Y98

Bighorn sheep (BHS) populations have been reported to experience high levels of morbidity and mortality following infection with <i>Mycoplasma ovipneumoniae</i>. This contrasts with the subclinical presentation in domestic sheep (DS). Understanding this difference requires baseline knowledge of pre- and post-infection immune responses of both species. The present study identifies differences in leukocyte phenotypes between adult BHS and DS before and after intranasal inoculation with 1 × 10⁸<i>Mycoplasma ovipneumoniae</i>. Prior to inoculation, BHS were confirmed to have a higher abundance of leukocyte CD14 and serum concentrations of IL-36RA. In contrast, DS had a higher leukocyte abundance of CD16 in addition to previously observed integrin markers and CD172a, as well as greater serum TNF-α concentrations. Within 15 days of inoculation, BHS displayed signs of mild respiratory disease and <i>M. ovipneumoniae</i> DNA was detected on nasal swabs using a quantitative PCR; meanwhile, DS exhibited few to no clinical signs and had levels of <i>M. ovipneumoniae</i> DNA below the standard curve threshold. Immunologic markers remained relatively consistent pre- and post-inoculation in DS, while BHS demonstrated changes in the peripheral leukocyte expression of CD172a and CD14. Circulating serum IL-36RA decreased and CXCL10 increased within BHS. These findings highlight significant differences in cellular immunity between BHS and DS, raised and housed under similar conditions, prior to and following inoculation with <i>M. ovipneumoniae</i>.