Secretion of immunoglobulins and plasma proteins from the colonic mucosa: anin vivostudy in man

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Abstract
There are no available data on immunoglobulins and albumin outputs into the normal human colon. We thus measured the intracolonic secretion rates of IgA, IgG, IgM, secretory component (SC) and plasma proteins (albumin (Alb), orosomucoid (Oro), transferrin (Transf) and alpha 2-macroglobulin (alpha 2-M)). Using a pancolonic perfusion technique in 10 healthy volunteers (six females, four males, mean age 24 years), concentrations and outputs of Alb, immunoglobulins, SC, Oro, Transf and alpha 2-M were measured in the rectal effluents by immunoradiometric assay. Monomeric (m) and polymeric (p) IgA distribution was analysed by sucrose density ultracentrifugation. The secretion of polymeric IgA (p-IgA) was 153 micrograms/min, i.e. 220 mg/day, exceeding that of other immunoglobulins (m-IgA 8.5 micrograms/min; IgG 33.5 micrograms/min; IgM 17 micrograms/min) and of non-immunoglobulin proteins (Alb 104 micrograms/min; Oro 9 micrograms/min; Transf 7 micrograms/min; alpha 2-M 4.5 micrograms/min). p-IgA was entirely linked to SC (secretory IgA) and 12% of SC was in free form. About 62% of total IgA was IgA2. For each protein, a relative coefficient of excretion (RCE) was calculated (colon to serum concentration ratio expressed relative to that of Alb). The p-IgA, IgM and m-IgA RCE were 277, 6 and 2.2 times higher than the values predicted from their molecular weight. RCE of non-immunoglobulin proteins also exceeded the values expected from a passive seepage from the vascular compartment. The intracolonic clearance of Alb extrapolated to 24 h was only 3.7 ml/day. These results show the high local production and/or the facilitated transport to the colonic lumen of p-IgA, and are in very good agreement with the distribution of plasma cells in the colonic mucosa.

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