CELLULAR BASIS OF THE GENETIC CONTROL OF IMMUNE RESPONSES TO SYNTHETIC POLYPEPTIDES : II. FREQUENCY OF IMMUNOCOMPETENT PRECURSORS SPECIFIC FOR TWO DISTINCT REGIONS WITHIN (PHE, G)-PRO--L, A SYNTHETIC POLYPEPTIDE DERIVED FROM MULTICHAIN POLYPROLINE, IN INBRED MOUSE STRAINS

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.