Sub-isotypic differences in the immunoglobulin G response to Lawsonia intracellularis in vaccinated, seropositive, and equine proliferative enteropathy-affected horses

In the horse, Lawsonia intracellularis infection results in equine proliferative enteropathy (EPE). While upwards of 100% of weanlings on an endemic farm may seroconvert, only a small percentage (approximately 5%) will develop clinical disease. Cell-mediated immune mechanisms likely play a role in resistance to L. intracellularis and the absence of a L. intracellularis-specific IFN-γ response has been associated with the development of EPE. The goal of this study was to determine whether protection from clinical EPE is associated with the induction of a systemic IgG sub-isotypic response consistent with a Th1-type cytokine response. To describe their L. intracellularis/EPE status, horses enrolled in this study were placed into one of three categories: seropositive-only, vaccinated, and presumptive clinical EPE. An existing ELISA method was modified to detect L. intracellularis-specific IgG(a), IgG(b), and IgG(t) antibodies using the mouse anti-equine hybridomas CVS-48, CVS-39, and CVS-40, respectively. Additionally, the existing ELISA method was used to quantify total IgG antibodies specific for L. intracellularis for comparison between the groups. Total L. intracellularis-specific IgG was found to be significantly higher (p < 0.05) in presumptive clinical EPE cases (n = 21) when compared with seropositive (exposed but unaffected) (n = 36) and vaccinated horses (n = 27). Further, a similar pattern for IgG(a) was seen in that the presumptive clinical EPE horses had significantly more L. intracellularis-specific IgG(a) (p < 0.05) than the seropositive or vaccinated horses. With IgG(b), however, the vaccinated horses had significantly more IgG(b) (p < 0.05) than the presumptive clinical or seropositive horses. No L. intracellularis-specific IgG(t) was detected in samples from any of the groups. While the results presented here with respect to IgG(a) response in the presumptive clinical EPE group were expected, a higher concentration of IgG(a) was anticipated in the seropositive horses that failed to develop clinical disease as well as in the vaccinated horses. Future work utilizing newer reagents against a broader range of equine IgG sub-isotypes may provide additional information once these become commercially available.