Serum immunoglobulin and soluble IL-2 receptor levels in small intestinal overgrowth with indigenous gut flora

Murine studies have demonstrated that the presence of indigenous gut flora is crucial for the induction of systemic immune hyporesponsiveness to antig ens initially encountered within the gastrointestinal lumen. This study inv estigated whether increased titers of such flora, as occur in human small i ntestinal bacterial overgrowth, may be associated with increased suppressio n of systemic immune responsiveness and the possible relation between syste mic and mucosal immunity in this setting. Serum total immunoglobulin (Ig), immunoglobulin subclass, and soluble interleukin-2 receptor levels and lami na propria IgA plasma cell counts were determined in 50 consecutive subject s with (N = 30) and without (N = 20) small intestinal bacterial overgrowth. Luminal IgA levels were measured in 35 of these subjects. Serum concentrat ions of IgG(3), but not of other immunoglobulin isotypes or soluble interle ukin-2 receptors, were significantly reduced in subjects with bacterial ove rgrowth (P < 0.0005). Small intestinal lamina propria IgA plasma cell count s (P < 0.0005) and luminal IgA concentrations (P = 0.001) were significantl y increased in this group. Serum IgG, levels were significantly inversely c orrelated with luminal IgA levels (P < 0.01) and fell below the lower limit of normal (0.41 g/liter) in 17/30 (56.7%) subjects with bacterial overgrow th compared to 1/20 (5.0%) subjects without (P < 0.0005). These findings do cument an association between small intestinal bacterial overgrowth with in digenous gut flora and reduced serum IgG, reactivity in humans, possibly vi a an interaction with mucosa-related immunoregulatory mechanisms. The possi bility of underlying small intestinal bacterial overgrowth should be consid ered in patients with serum IgG(3) deficiency, especially those with compat ible symptoms and/or known predisposition.