I Percent Injected Label Recovered from Recipient Organs After Transfer of Mesenteric and Peripheral Node Cells

Immunoglobulin A (IgA)-secreting plasma cells beneath secretory epithelia probably represent the terminal differentiation stage of lymphocytes originating in Peyer's patches. Here, it is postulated (1), lymphocytes acquire a commitment to IgA synthesis. They proceed to the mesenteric lymph nodes (MN), 1 where they divide and differentiate into plasmablasts, then emigrate via the thoracic duct and blood to the lamina propria, principally of the small intestine, where they lodge and undergo terminal differentiation to IgA-producing plasma cells. Several types of evidence implicate mesenteric nodes as the proximate source of IgA-secreting plasma cells in mucous membranes. Among them are these observations (2-4): (a) A subpopulation of the DNA-synthesizing large lymphocytes of rat and mouse mesenteric nodes homes to the lamina propria of the small intestine shortly after intravenous transfer into syngeneic recipients; mesenteric node blasts are unique in this capacity, which is not manifested by the bulk of resting cells nor by blasts from other lymphoid organs including Peyer's patches. (b) IgA-bearing B cells are relatively abundant in mesenteric nodes. (c) Most of the homing cells bear surface Ig, though the particular class of Ig has not hitherto been unequivocally identified. (d) Many of the homing cells differentiate into IgA-secreting plasma cells within 24 h of transfer. Despite this suggestive evidence, there is as yet no formal proof that the precursors of the IgA-producing plasma cells in the lamina propria are already committed to IgA synthesis before leaving the mesenteric nodes. We now report direct evidence that homing B blasts bear surface IgA while still resident in MN. Furthermore, the homing cells themselves produce the IgA which they bear, i.e., it is not passively adsorbed.