DBA/1 mice are high responders to the (Phe, G) determinant of the synthetic polypeptide (Phe, G)-Pro--L, whereas SJL mice respond well to the Pro--L region of this macromolecule (6). In order to determine whether the phenomenon described above is related to the number of antigen-sensitive units detected for both specificities, and whether responses to these determinants can be transferred independently, graded and limiting inocula of spleen cells from SJL, DBA/1, and F1 donors were injected into X-irradiated, syngeneic, recipient mice with (Phe, G)-Pro--L.

By this approach, one antigen-sensitive unit specific for (Phe, G) was detected in 1.7 x 106 and 8.5 x 106 spleen cells from immunized and nonimmunized DBA/1 donors, respectively. In contrast, one (Phe, G) relevant precursor was detected in 20 x 106 SJL spleen cells, irrespective of whether the donors had been immunized. On the other hand, for the Pro--L specificity, one limiting splenic precursor was found in 1.3 x 106 and in 3.4 x 106 cells for immunized and nonimmunized SJL donors, respectively; whereas one response unit was estimated for this determinant in 9.4 x 106 and in 38 x 106 spleen cells from immunized and nonimmunized DBA/1 mice. The findings reported here indicate that the phenotypic expression of the genetic control(s) for immune responsiveness to different immunopotent regions of (Phe, G)-Pro--L is directly correlated with the number of immunocompetent response units detected in two inbred mouse strains.

In the spleens of immunized F1 donors, similar frequencies of one limiting precursor in 3.0 x 106 and in 2.8 x 106 cells were detected for (Phe, G) and Pro--L, respectively. The results of a chi-square test for independence of (Phe, G) and Pro--L responses in F1 animals is compatible with the hypothesis that the transferred spleen cells limiting the response to (Phe, G)-Pro--L are restricted to generate antibodies specific for only one of the two determinants of this macromolecule.

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