F1 mice make two species of antigen-specific, parental haplotype-restricted, T-helper factor whose restrictions correspond to the phenotype of the I-A determinants that they bear.

Antigen-specific T-helper factor (ThF) bears I-A determinants and is I-A restricted in its action. This I-A restriction may be explained by ThF binding, and hence approximating antigen to its own I-A determinants, thereby facilitating recognition by the T cell (I-A presentation theory), or by a recognition site for I-A on the ThF which approximates the antigen to I-A+ antigen-presenting cells (I-A recognition theory). When ThF is produced in F1 mice there may be a dissociation between the I-A phenotype of the ThF and the I-A restriction of any recognition site for I-A. In practice, ThF is made by spleen cells from (CBA x B10)F1 [(H-2k x H-2d)F1] mice pretreated with cyclophosphamide (100 mg/kg) and immunized with picrylated parental spleen cells intravenously (3 x 10(7)). This procedure produces haplotype-restricted contact sensitivity (corresponding to the parental cells) but the unfractionated F1 ThF does not show haplotype-specific restriction in its action. In fact, the F1 mice produces two species of ThF, each bearing determinants of the I-A molecule of one parental haplotype (k or d). When the two species were separated with monoclonal anti-I-Ak and anti-I-Ad antibodies, the genetic restriction in their action corresponded to the phenotype of the I-A determinants that they carried. In a further experiment, F1 ThF was split into its constituent antigen-binding (Ag+) and antigen non-binding (Ag-) chains by reduction, and the two species of Ag- chains separated with monoclonal anti-I-A antibodies. After complementation with the Ag+ chain, the two species of Ag- chains showed genetic restriction in their action that corresponded to the phenotype of the I-A determinants that they carried. These findings support the I-A presentation hypothesis that antigen-specific ThF acts by approximating antigen to its own I-A determinants, and hence facilitates recognition by I-A-restricted T cells.