Immunization with peptide heteroconjugates primes a T helper cell type 1-associated antibody (IgG2a) response that recognizes the native epitope on the 38-kDa protein of Mycobacterium tuberculosis

Heteroconjugates were prepared by covalently linking a peptide (M) bearing the 38.G epitope from the 38-kDa protein (residues 350-369) of Mycobacterium tuberculosis, to peptides containing T-cell binding ligands (TCBL) or keyhole limpet hemocyanin (KLH). BALB/c mice were immunized with the heteroconjugates using incomplete Freund's adjuvant (ICFA) supplemented with muramyl dipeptide. The resultant antisera were analyzed by enzyme-linked immunosorbent assay (ELISA) procedures. Antibody titers and serotype were used as indicators of involvement and modulation of T-helper (Th) activity. Initial immunization and challenge with the TCBL-containing heteroconjugates did not elicit specific antibody to the M peptide (38.G epitope); however, a heteroclitic response was generated. The peptide conjugated to KLH (M-KLH) elicited specific anti-M antibody. Specific antibody was elicited following challenge of animals primed with the TCBL heteroconjugate and then challenged with M-KLH. The response was unlike unprimed mice, or animals primed with peptide M or control heteroconjugates. The antibody subtypes produced against peptide M were consistent with a Th1 response (IgG2a) for the heteroconjugates containing major histocompatibility complex (MHC) class I TCBL and Th2 responses (IgG1) for those containing MHC class H T-cell binding ligands or KLH. Immunoblot analysis indicated that these TCBL heteroconjugate-associated antibodies recognized the 38.G epitope in the 38-kDa protein of M. tuberculosis (reduced-SDS-denatured protein). However, unlike the TCBL-heteroconjugate-induced antibodies, the anti-peptide 38.G antibodies elicited from M-KLH-primed animals were unable to recognize the epitope in extracts containing the native protein from sonicated bacteria. ELISA studies indicated that the heteroconjugate-induced IgG2a antibodies recognized the 38.G epitope on the native protein in sonicates of various Mycobacterium sps. The results for M-peptide-TCBL heteroconjugates mimic a natural rechallenge and suggest that memory cells were elicited by the initial priming immunization. The implications of preferential stimulation of specific Th cell subsets, selective production of defined IgG subclass antibodies, and antibodies reactive against a native tuberculosis disease-related protein from different strains are discussed.