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 (ThE) 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 × B10)F₁ [(H-2^k × H-2^d)F₁] mice pretreated with cyclophosphamide (100 mg/kg) and immunized with picrylated parental spleen cells intravenously (3 × 10⁷). 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 ThE, 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-A^k and anti-I-A^d antibodies, the genetic restriction in their action corresponded to the phenotype of the I-A determinants that they carried. In a further experiment, F₁ ThE 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 ThE acts by approximating antigen to its own I-A determinants, and hence facilitates recognition by I-A-restricted I cells. [ABSTRACT FROM AUTHOR]