Fifteen T-cell clones, derived from LNCs taken from mice immunized with mRBP-3 1-16, were activated with mRBP-3 1-16 and mRBP-3 1-16 COOH or one of each of the nine mRBP-3 1-16 APLs. Flow cytometric analysis of 14 of the clones with a panel of eight anti-Vβ antibodies (Vβ2, -3, -5, -6, -7, -8, -11, and -14) and anti-CD4 was performed. All clones tested stained positive for CD4. Clones stained positive for Vβ3, -6, -7, or -11 (data not shown), indicating that the T cells were drawn from a polyclonal population and not restricted to a limited TCR repertoire. We did not find a significant association between Vβ expression and patterns of reactivity. We then considered two possibilities to explain the reactive patterns of these clones: they might recognize the mRBP-3 1-16 peptide bound in a single register, in which case the response patterns of APLs would be predicted to show some uniformity, or the clones may be divided into some that recognize peptide bound in one register and others that are stimulated by peptide bound in a second register, in which case we would predict no uniformity in APL activation. We found that there were at least five distinct patterns (shown by two or more clones) of proliferation and IFN-γ and IL-2 production after stimulation of the T-cell clones. Four of these are illustrated
(Fig. 5) , the fifth group of clones recognized mRBP-3 1-16 and F6Y only. Clone 1B10, and one other, proliferates above background and produces IFN-γ and IL-2 after stimulation with mRBP-3 1-16, mRBP-3 1-16 COOH, F6Y, and L12N
(Figs. 5A and 5E) . The second pattern, shown by 1F7 and four other clones, was a low-level response to mRBP-3 1-16 COOH activation, activation with F6Y that was greater than with mRBP-3 1-16 (an average of more than 50 × 10
3 cpm higher than activation with mRBP-3 1-16) and a response to Q7S. 1F7 alone also responds to D13I, although this APL induced much less IFN-γ than the other ligands
(Figs. 5B 5F) . IFN-γ production after F6Y stimulation was also higher than that after stimulation with mRBP-3 1-16
(Fig. 5Bii) . Clone 1F3 and one other clone were the most broadly cross-reactive and showed equivalent levels of proliferation after activation with mRBP-3 1-16, mRBP-3 1-16 COOH, F6Y, Q7S, P8A, and L10Q and more variable responses to L12N
(Figs. 5C 5G) . Although L10Q reproducibly induced proliferation, this ligand did not elicit equivalent levels of IFN-γ and IL-2 compared with the other peptides. The final pattern, represented by T-cell clone 1E11 and shared with one other clone, was the response to F6Y and a low but clearly reproducible level of proliferation in response to stimulation with S9Q
(Figs. 5D 5H) . There was no detectable IL-2 production from this T-cell clone, but IFN-γ production mirrored the level of proliferation, with a low level observed as a result of S9Q activation
(Fig. 5Dii) .