title: Intersite helper function of T cells specific for a protein epitope that is not recognized by antibodies.
abstract: Humoral responses to a protein require T-B cell communication for B cell activation by T cells. Previous studies from this laboratory have mapped the T and B cell recognition sites (epitopes) on sperm-whale myoglobin (Mb) and several other proteins. It was found that, five of six regions on Mb recognized by T cells are also recognized by B cells (i.e. antibodies). There is, however, one region (E6) residing within Mb residues 61-77, that is recognized only by T cells and to which no antibody (Ab) responses are detectable. To investigate the function of this exclusive T cell epitope, we established, from E6-primed BALB/c mice, an E6-specific T cell line (T(e6)) which comprised Th2-type cells. These T cells provided help in vitro to B cells from Mb-primed BALB/c mice and activated them to produce anti-Mb Abs of the IgM (58.2%) and IgG (41.8%) isotypes. The helper activity of T(e6) cells was dependent on the concentration of the challenging Ag (intact Mb or peptide E6) in culture. Action of soluble factors released from E6-activated T(e6) cells on B(mb) cells led to low production of anti-Mb Abs, suggesting that activation of the B cells was more dependent on their contact with T cells. Mapping of the epitope recognition of the anti-Mb Abs produced in vitro by B(mb) cells on activation by T(e6) revealed that this activation was not general to all antigenic regions recognized by anti-Mb Abs in BALB/c mice. E6-specific T cells caused in vitro activation and differentiation of B(mb) cells into plasma cells that secreted anti-Mb Abs directed, in decreasing order, against the following Mb regions: E4 (107-120) > E3 (87 - 100) > E1 (10 - 22). Little or no Ab responses could be detected against peptides E2 (50 - 62), E5 (141 - 153) and E6 (61 - 77). With B cells of peptide-primed BALB/c mice, T(e6) cells activated strongly E4-, E3- or E1, and only very slightly E2- or E6-, primed B cells to secrete Abs against the correlate peptide, but failed completely to activate E5-primed B cells. The results show that a protein T cell epitope, to which no Abs are detectable, plays an active role in B cell responses against other epitopes within the same protein.
authors.affiliation: Verna and Marrs McLean Department of Biochemistry, Baylor College of Medicine, Houston, TX 77030, USA.
authors.forename: J S
authors.initials: JS
authors.lastname: Rosenberg
authors.forename: M Z
authors.initials: MZ
authors.lastname: Atassi
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000906:Antibodies; D001402:B-Lymphocytes / Q000276:immunology; D002460:Cell Line; D017077:Culture Media, Conditioned; D000939:Epitopes / Q000737:chemistry / Q000235:genetics; D005260:Female; D007114:Immunization; D066298:In Vitro Techniques; D008213:Lymphocyte Activation; D008211:Lymphocyte Cooperation; D051379:Mice; D008807:Mice, Inbred BALB C; D009211:Myoglobin / Q000737:chemistry / Q000235:genetics / Q000276:immunology; D010446:Peptide Fragments / Q000737:chemistry / Q000235:genetics / Q000276:immunology; D011506:Proteins / Q000737:chemistry / Q000235:genetics / Q000276:immunology*; D018418:Th2 Cells / Q000276:immunology*
chemical_list: D000906:Antibodies; D017077:Culture Media, Conditioned; D000939:Epitopes; D009211:Myoglobin; D010446:Peptide Fragments; D011506:Proteins