Synergy among lymphoid cells mediating the graft-versus-host response. I. Synergy in graft-versus-host reactions produced by cells from NZB-Bl mice.

The ability of spleen cells from young (3 month) and old (1 yr) NZB mice to induce GVH reactions in newborn C57BL/6N mice was compared quantitatively using the Simonsen spleen assay. Young NZB cells were five times more reactive than cells from older mice. The minimum number of cells producing detectable reactions was 2 x 10(6) for the young and 10 x 10(6) for the old. Young and old cells combined and injected together produced GVH reactions quantitatively similar to those obtained with inocula composed of young cells alone. Mixtures of two cell populations producing no detectable reactions when injected separately into different recipients (1 x 10(6) young cells and 4 x 10(6) old cells) produced reactions approximately equal to those obtained with 5 x 10(6) young cells. As few as 0.25 x 10(6) young cells were sufficient to effect a reaction when combined with 4.75 x 10(6) old unreactive cells. Viability of both cell populations was essential for GVH reactivity. This evidence of synergy in GVH reactions indicates that old NZB spleen cells can be rendered immunologically more reactive in the presence of a normally reactive population.