Developmental changes in the capacity for mucosal immunoglobulin production and secretion in the intestines of growing calves.

Neonatal calves predominantly rely on colostral IgG for the passive transfer of immunity; however, little is known about their intrinsic capacity for mucosal immunoglobulin production and the developmental changes associated with their growth. To elucidate the developmental trajectory of mucosal immunity, we investigated changes in mucosal immunoglobulin concentrations and the expression levels of genes involved in immunoglobulin production and secretion across different growth stages in calves. The results demonstrated that fecal IgG and IgM levels exhibited transient peaks at 1 week of age according to smooth spline analysis, followed by sharp decreases, whereas IgA levels remained relatively stable and became the predominant isotype after 4 weeks. Gene expression analysis and immunohistochemistry revealed the localized expression of immunoglobulins in the intestinal mucosa, particularly IgA, which gradually increased with calf growth. The secretion of IgA is also thought to be facilitated by the upregulated expression of PIGR, a gene encoding the IgA transporter whose expression levels increase with calf growth. In contrast, the levels of plasma cell-recruiting chemokines and their receptors were not increased. These results suggest an important role for IgA in the mucosal defense system of the calf intestine, indicating its pivotal function in maintaining gut health following the clearance of colostral IgG.