Mice inoculated with brain homogenates from multiple sclerosis (MS) cases showed marked changes in their leukocyte differential counts, with a decrease in per cent polymorphonuclear neutrophils (PMN) and an increase in the per cent lymphocytes. These changes were based upon an absolute decrease in the number of circulating PMN. The decrease in PMN percentages was apparent at 16 hr after infection and persisted for at least 11 months. The factor responsible for the decrease in PMN was (a) recoverable from 12 hr to 8½ months after inoculation, (b) present in human brain homogenate at a concentration of 3 x 1012, and (c) between 25 and 50 nm in diameter. Inoculation of 100 units of factor into mice and subsequent titration showed that the factor had undergone a net increase in the mouse of at least 109-fold. The factor causing the PMN decrease was found in all MS material thus far tested: three brains, one spleen, three sera, and two cerebrospinal fluid (CSF) from nine cases of MS. The factor was not found in normal human material that included two brains, one spleen, two sera, and two CSF.

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