Role of pokeweed mitogen spleen conditioned medium in regulating haematopoiesis in long‐term human marrow cultures

The modulatory effect of pokeweed mitogen spleen conditioned medium (PWM‐SCM) on the growth of myeloid progenitors (CFU‐C) and on the composition of the stromal layer in human bone marrow long‐term cultures was studied. In the presence of PWM‐SCM there was enhanced CFU‐C growth in the adherent layer throughout 6 weeks of incubation, with 2‐fold, 2‐fold and 3.4‐fold increases during wk 2, 4 and 6 respectively (p < 0.05). In the non‐adherent layer of control cultures, however, there was a rapid decline in haematopoiesis which could not be offset by PWM‐SCM. The differentiation pattern of CFU‐C (entailing monocyte — macrophage progenitors CFU‐M, granulocytic progenitors CFU‐G or monocyte — granulocyte progenitors CFU‐GM) was not altered by PWM‐SCM. Fewer lipid‐containing cells developed in the adherent layer of cultures treated with PWM‐SCM than in control cultures, but the distribution pattern of monocytes — macrophages or granulocytes was not affected. These results suggest that in long‐term cultures of human marrow cells there may be extensive haematopoiesis in the presence of PWM‐SCM. Whether this is a direct effect on progenitor cells of stimulating factors, or rather of accessory cells in the adherent layer, remains to be clarified.