Human T-lymphotropic virus type 1 peptides in chimeric and multivalent constructs with promiscuous T-cell epitopes enhance immunogenicity and overcome genetic restriction

Abstract

Conventional strategies of viral peptide immunizations often elicit low-affinity antibody responses and have limited ability to elicit immune responses in outbred animals of diverse major histocompatibility (MHC) haplotypes. This genetically restricted T-cell-stimulatory activity of peptides is a serious obstacle to vaccine design. However, the use of promiscuous T-cell epitopes may circumvent this problem. Promiscuous T-cell epitopes from tetanus toxin (amino acids [aa] 580 to 599) and the measles virus F protein (aa 288 to 302) that bind to several isotypic and allotypic forms of human MHC class II molecules have been identified and have been used in highly immunogenic constructs to overcome haplotype-restricted immune responses. Chimeric and beta-template peptide constructs incorporating known human T-lymphotropic virus type 1 (HTLV-1) B- and T-cell epitopes from the surface envelope protein gp46 (SP2 [aa 86 to 107] and SP4a [aa 190 to 209]) and promiscuous T-cell peptides were synthesized, and their immunogenicities were evaluated in both rabbits and mouse strains of divergent haplotypes (C3H/HeJ [H-2k], C57BL/6 [H-2b], and BALB/c [H-2d]). In addition, peptide preparations were structurally characterized by analytical high-performance liquid chromatography, mass spectrometry, and circular dichroism. In contrast to their linear forms, the chimeric constructs of both the SP2 and SP4a epitopes displayed alpha-helical secondary structures. Immunogenicity of the peptide constructs was evaluated by direct and competitive enzyme-linked immunosorbant assay (ELISA), as well as by radioimmunoprecipitation, syncytium inhibition, and antigen-induced lymphocyte proliferation assays. Immunization with the SP4a peptide without conjugation to a carrier protein produced antibodies specific for SP4a in two mouse strains (C3H/HeJ and C57BL/6). However, BALB/c mice failed to respond to the peptide, indicating that the T-cell epitope of the SP4a sequence is MHC restricted. In contrast, the chimeric constructs MVF-SP2 and SP4a-measles virus F protein were highly immunogenic, producing elevated ELISA titers after only two immunizations. Elicited antibodies recognized native forms of gp46 in ELISAs and radioimmunoprecipitation assays, as well as inhibited HTLV-1-mediated syncytium formation. In addition, chimeric constructs were effective at induction of lymphocyte proliferation to the T-cell epitope, SP4a, in each strain of immunized mice. Our data demonstrate that the antibody response to retroviral peptides is enhanced by promiscuous peptide constructs, in part because of the ability of such constructs to promote appropriate secondary structural forms of viral epitopes. In addition, these constructs promote virus-specific helper T-cell responses, thereby overcoming genetically restricted immune responses to the synthetic peptides.