Contribution of serum IgA to intestinal lymph IgA, and vice versa, in minipigs

Immune cells in pig gut lymph are rather well studied, but data on gut lymph immunoglobulins and their origin are nonexistent. Such data are important to understand the interplay between pig systemic and intestinal immunity as a basis for vaccination studies. In some species, gut lymph contributes much to plasma IgA, but apparently not in humans. To estimate the contributions of pig serum IgA to intestinal lymph IgA and vice versa, concentrations of IgA, IgG, IgM, albumin, haptoglobin, C3 and α2-macroglobulin were measured by radial immunodiffusion in paired porcine intestinal lymph and serum samples. All proteins, except IgA, had lymph/serum ratios (< 1.0) inversely related to their size, depending on passive diffusion from serum. The mean lymph/serum ratio of IgA was 2.2 instead of an expected 0.50 or 0.65 (dimer or monomer, respectively), indicating that of the IgA in gut lymph, 22.7 or 29.5% came from serum, vs 77.3 or 70.5% from the intestine. Percentage of polymeric IgA, measured by gelfiltration and corrected radial immunodiffusion, was 64.3% in porcine mesenteric lymph and 47.3% in serum. As the pig plasma volume and daily gut lymph flow into circulation were known, it could be calculated that roughly 31% of the total plasma IgA originated daily from local intestinal synthesis, reaching blood via mesenteric lymph.