Transformation of hemopoietic stem cells by phytohemagglutinin (PHA) II. Mechanism of action.

It has long been known that phytohemagglutinin (PHA) stimulates transformation and growth of immune competent lymphocytes. Lymphoid cell colonies have previously been shown in the spleen with PHA treated lymph node cells from donor mice were injected into irradiated mice. This communication reports the results of in vivo effects of PHA stimulation on agar colony forming units and spleen colony forming units (CFU). C3H/Hej mice injected with 1 ml of PHA-M were found to secrete colony stimulating factors (PHA-CSF) which lead to an increase in the number of agar colony forming units (granulopoiesis). Serum obtained on day 6 after PHA injection showed enhanced granulopoiesis which was four times higher than in the controls. It was found that 1.0 ml and 0.15 ml of PHA-CSF were more effective in promoting the growth of agar colony forming units in agar plates. Intravenous injection of PHA increased the number of spleen-as well as agar colony forming units (CFU). Day 3 appeared to be optimal for in vivo effects of PHA on granulopoietic cells. An increase in the colony forming units was obtainable when previously treated bone marrow and spleen cells taken on day 3 were injected into irradiated mice. There was a fourfold increase of the colony forming units (CFUs) in the spleen and a twofold one in bone marrow cells. As compared to the controls, the increase in agar colony forming units (CFUc) of bone marrow was fourfold and that of the spleen tenfold. On day 5 some effect was still noticeable but it was lower than on day 3. On weight and cell count basis it was found that on day 6 PHA had a significant in vivo effect on the spleen. On the basis of our findings it can be concluded that PHA supports the survival of transplanted stem cells by stimulating their differentiation into unipotent erythroid progenitor cells. It may also be concluded that PHA activates the immune competent stem cells (mostly T lymphocytes) and displays a supporting function for a better stem cell survival and differentiation into the erythroid progenitor cells. The activated lymphocytes secrete a colony-stimulating like factor which stimulates granulopoiesis and also helps in the differentiation of the stem cells.