The gut‐associated lymphoid system: nature and properties of the large dividing cells

Abstract The nature of the lymphoid cells and blasts (i.e. cells labeled in vitro with [3H]‐thymidine) of the Peyer's patches (PP), mesenteric lymph nodes (MLN) and thoracic duct lymph (TDL) has been studied by combined immunofluorescence and radioautography, using antisera directed against T (MSLA) or B (MBLA) cell antigenic determinants, or against various Ig chains. The migration pattern of these various cell populations after transfer to syngeneic mice or rats was determined by radioactivity counting and radioautography and the nature of the homed cells was studied by combined immunofluorescence and radioautography. B and T blasts from MLN and TDL have, in contrast to blasts from the peripheral lymph nodes, a marked tendancy to home in the gut, and migrate as well in a graft of fetal intestine as in the recipient's own gut, indicating that this selective accumulation is not the result of a mechanism recognizing antigen absorbed from the gut lumen. The gut‐homing B blasts are cells bearing surface IgA (sIgA) and containing intracellular IgA, which transform into IgA plasam cells in the lamina propria. Evidence is discussed that the progenitors of the intestinal IgA plasma cells are proliferating sIgA cells from the PP geerminal centers, which migrate via lymphatics to MLN and TDL and mature during this migration, acquiring intracellular IgA chains as well as a gut‐homing mechanism which might be related to this increased IgA synthesis. The gut‐homing T blasts migrate both in the lamina propria and the intestinal epithelium. Most, if not all, the intraepithelial lymphocytes of the gut mucosa appear to be T lymphocytes derived from rapidly dividing cells. Evidence is presented for the existence in the lymphoid system of two types of T blasts, one that is circulating and has a marked tendancy to home to the gut, and a second that does not home to the gut and may be sessile.