Intravenous injection into the rat of the lymphocytosis-promoting factor (LPF) caused a striking leukocytosis characterized by predominant small lymphocytes. The lymphocytosis appeared as early as 3 hr after the administration of LPF.
The mechanism of the lymphocytosis with LPF in the rat was investigated under the current knowlege of the lymphocyte recirculation by the thoracic duct drainage technique.
The lymph flow and the lymphocyte output from the thoracic duct drainage were almost completely blocked several hours after the LPF injection without any period of apparent augmentation in lymphocyte output, while the number of peripheral lymphocytes continued to increase as did the LPF-inoculated rats having no thoracic duct cannula. The evidence suggested that the entry of lymphocytes from the lymphatic tissues into the blood stream via the so-called direct route was enhanced by the LPF administration. The size of increase, however, was so small that the lymphocytosis was not attributable to the increase in the lymphocyte output via the direct route. On the other hand, the lymphocytes treated in vivo or in vitro with LPF did not migrate from the blood stream into the lymph nodes when transfused into normal rats.
These are the evidences to show that the lymphocytosis induced by LPF is due to the modified recirculation of lymphocytes as a result of direct effect of LPF on small lymphocytes. The affected lymphocytes lose the capacity to go back from the blood stream to the lymph nodes. There is no evidence of increase in the rate of entry of lymphocytes from the lymph nodes into the blood stream.