Relationship between large and small tumor-binding cells in the spleen and bone marrow

By employing a distinctive feature of natural killer (NK) cells, i.e., spontaneous target cell binding, the present study aimed to follow a relatively large cohort of target-binding cells (TBC), subdivided according to size and the presence of the radioautographically labelled nucleotide, tritiated thymidine, incorporated during a 1- or 6-hour exposure period. Labelling among all TBC in the spleen was 5% by 1 h after a single injection of the isotope and this value did not change significantly after 6 h of isotope exposure. There was an insignificant increase in the labelling index of spleen-localized, labelled, large TBC throughout and beyond the isotope exposure period for at least 48 h, concomitant with a significant increase in the labelling index of spleen-localized, small TBC during the same period. In the bone marrow, small TBC showed only 2% labelling by 1 h after a single injection of isotope, while 21% of large TBC in that organ were synthesizing DNA during the same period. The results are consistent with a precursor-product relationship between large and small TBC within the bone marrow but not the spleen, and with a bone marrow-to-spleen migration of small (+/- large) TBC. Moreover, a minor population of large TBC was detected in the spleen with kinetic characteristics distinct from those of the bone marrow.