The distribution of large dividing lymph node or thoracic duct lymph cells, labeled in vitro with 3H-thymidine, was studied in syngeneic recipient rats after intravenous injection. In most experiments the donor rats had been immunized with Bacillus pertussis 4 days earlier, but in some instances cells from nonimmunized donors were used. In smears, the labeled donor cells had the appearance of large lymphocytes or large pyroninophilic cells. By electronmicroscopy, the majority of labeled donor cells were seen to have only scanty endoplasmic reticulum.

It was found that the labeled cells rapidly "homed" to lymphoid tissue and recirculated in the recipient, in a fashion resembling that of small lymphocytes. However, the distribution of labeled cells was found to depend upon the source of the donor cells. Cells from mesenteric lymph nodes or thoracic duct lymph showed a marked preferential accumulation in lymphoid tissue within or adjacent to the intestine, whereas cells from peripheral nodes accumulated preferentially in peripheral lymph nodes. Cells from any of these sources showed an equal tendency to accumulate in the white pulp of the spleen.

Suspensions of small lymphocytes, labeled in vitro with 3H-uridine, did not display a similar tendency to localize preferentially in lymphoid tissue in certain regions.

It was also found that large dividing lymph node cells from donors immunized with an antigen (2,4-dinitrophenyl-bovine gamma globulin (DNP-BGG) or B. pertussis) showed a greater tendency to accumulate in a recipient lymph node containing that antigen than in the contralateral node.

It was not determined whether the selective accumulation of large dividing lymphoid cells from different sources in lymphoid tissue of different regions in recipients was due to an antigen recognition mechansim or was the result of two different populations of cells with different "homing" mechanisms.

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