In 1963, a strain of SHR rats which developed spontaneously hypertension was isolated from Wistar rats by Okamoto and Aoki (JPN CIRC J 27: 282, 1963). In preliminary studies to investigate T-lymphocyte markers in rats we noted that SHR rats had reduced numbers of rosette forming cells in their thymuses when compared to other rat strains (CELL IMMUNOL 27: 52, 1976). Additional studies were undertaken to more clearly determine if additional impairment of immunological functions existed in this

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... strain with hypertension. The percentage of rosette forming cells in the thymus and in the spleen of the eight strains was tested. In seven rat strains, about 60% thymus cells formed rosettes with guinea pig erythrocytes, whereas with the SHR rat strain only 18% of thymus cells formed rosettes. W/Mk rats, the original colony of the SHR rats, had 63% rosette formation. None of the strains showed any rosette forming cells in their spleens. The number of rosette forming cells progressively decreased with age. The blastogenesis of SHR rat lymphocytes to PHA and Con A was less than one fifth to that of the other rat strains including W/Mk rats, the original colony of SHR rats. Lymphocytes from 8-month-old SHR rats had significantly lower blastogenic responses than did lymphocytes of 2-monthold SHR rats. aFour month old rats were used . Table shows the results of antibody responses to SRBC of the SHR and other rat strains using the direct plaque-forming assay. Antibody responses of the 4 month old SHR rats was about one tenth to that of the other 6 rat strains. Delayed type hypersensitivity responses of SHR rats to SRBC were measured using the radioisotopic footpad assay. The footpad reaction of immunized SHR rats was significantly lower than that of immunized WKA rats. Ability of SHR rats to reject alloskin grafts was also investigated. The number of mean survival days of allogeneic skin from F344 rats was significantly prolonged when compared to WKA rats (data not presented). The thymuses of young SHR rats before developing hypertension had reduced numbers of immature T-lymphocytes which were detected by the rosette formation test with guinea pig erythrocytes, whereas the thymuses of other eight rat strains tested contained about 60% rosetting cells. The number of rosetting cells progressively decreased with age. The blastogenic responses to PHA of the SHR rat lymphocytes was depressed to less than one fifth when compared to those of other rat strains including W/Mk rats, the original colony of the SHR rats. Other cell-mediated immune responses, including delayed hypersensitivity, allograft rejections, and a cooperation of T-and B-lymphocytes to produce humoral antibody formation were significantly depressed when compared to those of other rat strains. 590