Control of in vivo differentiation of myeloid leukemic cells. iv. inhibition of leukemia development by myeloid differentiation‐inducing protein

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Abstract
It is shown that a 5‐day schedule of two injections per day of the myeloid differentiation‐inducing protein MGI‐2 inhibited the in vivo development of leukemia in SL and SJL/J mice with different syngeneic MGI+D+ clones of myeloid leukemic cells. With this schedule of treatment high levels of MGI‐2 were maintained in the serum for long periods. In contrast to these results with MGI‐2, the same schedule of injections of the myeloid growth‐inducing protein MGI‐1 did not affect the in vivo development of leukemia in mice with MGI+D+ myeloid leukemic cells, but stimulated normal myelopoiesis in the bone marrow. Different forms of MGI‐1 including MGI‐1M and MGI‐1G had different serum half‐lives, and the form of MGI‐1 with the shortest serum half‐life showed the smallest in vivo effect on normal myelopoiesis. MGI‐2 injections did not inhibit the in vivo development of differentiation‐defective WEHI‐3B myelomonocytic leukemic cells or YAC lymphoma cells. The results indicate that the in vivo inhibitory effect of MGI‐2 on the development of myeloid leukemia correlated with its differentiation‐inducing potential on the leukemic cells. It is concluded that this approach of inhibiting leukemia development by inducing differentiation should also be applied to human leukemic patients, whose cells have been shown to be inducible for differentiation in culture by human MGI‐2 or by other differentiation‐inducing compounds.

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