By using a cytotoxicity inhibition assay employing AKR anti-C3H thymocyte antiserum, we have determined the degree of expression of the thy-1 antigen in fractions of adult mouse brain. As expressed as cytotoxicity inhibitory capacity per mg protein with C3H whole brain arbitrarily assigned a value of 1.0, the following values were found: C3H cerebral cortex, 5.8; C3H cerebral cortex synaptosomes, 2.5: C3H whole brain myelin, 0.65; C3H cerebral cortex neurons, 0.16; and C3H cerebral cortex mitochondria, 0.10. Neither C1300 neuroblastoma cells nor any AKR neural fraction had detectable levels of thy-1.

The findings indicate that the thy-1 antigen is found mainly in mouse cerebral cortex and in synaptosomal fractions, whereas myelin fractions contain lower but perhaps significant amounts of thy-1. Cerebral cortex neurons, isolated by a method requiring a 90-min mild trypsinization at 37°C, did not display significant amounts of the thy-1 antigen.

These results lend themselves to further study in the area of differentiation and development of central nervous system components and in the area of central nervous system immunopathology.

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This work was supported by Grant MS-889-B2 from the National Multiple Sclerosis Society and Grant NS-05572-13 from the National Institutes of Health.

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