Studies have been made on the oligosaccharide residues of the alkali-stable carbohydrate-protein linkage of sialoglycopeptides derived from human erythrocytes. Four glycopeptides were isolated after alkaline borohydride treatment and Pronase digestion of MN-active sialoglycopeptides. The structure of one of these glycopeptides (GPIV) has been studied by sequential hydrolysis with specific glycosidases. Glycopeptide GPIV contained (per mol): 1mol of fucose, 1mol of sialic acid, 3mol of galactose, 3mol of mannose, 4mol of acetylglucosamine, 1mol of aspartic acid and fractional amounts of threonine, serine and glycine. The molecular weight of the glycopeptide was estimated to be 2330 by gel filtration. On the basis of glycosidase-digestion results, a tentative structure is proposed for the oligosaccharide moiety of glycopeptide GPIV.

A procedure has been investigated for sorting viable cells according to their DNA content. Cells are stained with the U.V. activated fluorochromes 4'6-diamidino-2-phenylindole (DAPI), Hoechst 33258 or Hoechst 33342, and sorted with a Fluorescence Activated Cell Sorter. Hoechst 33342 is a suitable vital stain for a variety of cell types. Hoechst 33258 and DAPI, however, are quantitative vital stains for CHO cells only. Cloning efficiency is unaffected by the sorting procedure, and these stains are not mutagenic at concentrations suitable for vital staining. Potential applications of this procedure to cell biology are discussed.

An extensive analysis of the class II (I-Ad)-restricted T cell repertoire for influenza hemagglutinin (HA) of the H3 subtype, elicited by natural infection, has shown that majority of CD4+ memory T cell clones focus on antibody-binding regions of HA, sites B and E, and are sensitive to the residue substitutions that have occurred in these regions during antigenic drift. The proliferative responses of CD4+ clones to synthetic peptides have identified T cell epitopes within site B, HA1 177-199 and HA1 182-199, and site E. HA1 56-76. The recognition specificity of T cell clones for antibody-selected mutant viruses, with single amino acid substitutions within these recognition sites identified residues 63, 189, 193 and 198 as being important for T cell recognition and thus established that BALB/c, CD4+ T cell clones were sensitive to the same substitutions known to abrogate BALB/c antibody recognition of the native HA. Our findings indicate extensive commonality of the B cell and T cell repertoires for HA, which may be relevant to an understanding of the immune pressures for antigenic drift, and, moreover, suggest that the antigen-specific B memory cell may be instrumental in selection of the peripheral T cell repertoire.

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An essential feature of the immune system is its ability to recognize and respond to a diverse spectrum of foreign antigens encountered in the environment. Two kinds of antigen specific receptors are used, the Ig receptor on B cells and the antigen-specific receptor on T cells. In contrast to Ig, which binds antigen directly, the T cell antigen receptor binds antigen only in association with self MHC-encoded gene products (1, 2). Despite this functional difference between T and B cell antigen recognition, recent molecular studies (3-6) on the T cell antigen receptor have shown considerable structural homology with Ig and similar mechanisms for the generation of antibody and T cell receptor diversity.At the cellular level, the diversity of Ig molecules generated at the DNA level is reflected by the ability of B cells to recognize an almost unlimited number of antigenic determinants (7-9). However, Th cells are thought to have a much more restricted repertoire (10). Studies with T cell clones specific for myoglobin (11, 12), lysozyme (13), and cytochrome c (14) have suggested that Th cells recognize a limited number of immunodominant carrier epitopes. The demonstration of T cell determinants distinct from antibody-combining sites (10, 15, 16) has been explained by the T cell requirement for antigen processing (17), which allows recognition of linear or unexposed determinants, in contrast to direct binding to exposed conformational determinants by B cells. However, in view of the extensive germline and combinatorial diversity demonstrated in genes coding for the variable region of the T cell receptor, it is difficult to explain the apparent limited number of T cell specificities so far demonstrated from studies of T cell clones specific for an individual antigen. The majority of studies with T cell clones have focused on synthetic antigens or conserved globular proteins. The use of an infectious agent such as influenza virus, which permits priming by infection rather than by the usual footpad route using adjuvant, may reveal a somewhat different pattern of T cell recognition.The immune response to influenza virus involves a T cell-dependent production of neutralizing antibodies directed against the hemagglutinin (HA) 1 surface glycoprotein (18,19). The HA molecule undergoes continuous antigenic variation as a result of point mutations in the virus genome (antigenic drift) which i Abbreviations used in this paper: HA, hemagglutinin; sup, supernatant.J. ExP. MED.

A panel of H-2k class II-restricted T helper (Th) clones were established from individual CBA mice intranasally infected with A/X31 influenza virus. In a study of their fine specificity, it was revealed that recognition of hemagglutinin (HA) by certain Th clones was dependent on conformational features of the molecule. Five HA-specific Th clones failed to recognize X31 mutant viruses R19 and R20 each with a His to Arg substitution at position 17 of HA1. This amino acid substitution affects the conformational stability of the molecule. Each of the 5 clones responded to purified HA but failed to recognize tryptic fragments of HA consisting of HA1 residues 28-328 (tops) and the remainder of the virus including residues 1-27 of HA1 (aggregate). A further 20 HA-specific clones responded to R19 and R20 mutants and to HA1 tops. The R19 and R20 negative clones also showed diminished proliferative responses to pH 5-treated X31. Low pH treatment results in an irreversIble conformational change which affects the integrity of the globular head region of the HA trimer. In addition, four of the five clones failed to recognize variant virus Eng-72 which has Arg to Gly substitution at position 208 in the interface antibody-binding region. This region is antigenically and structurally altered in tops and pH 5-treated X31. The results suggest that recognition of HA by certain Th clones is dependent on the three-dimensional structure of the molecule and that Th cells may recognize conformational determinants in addition to primary structure.

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A majority of I-Ad-restricted CD4+ clones elicited by influenza X31 (H3N2) virus infection, recognize a synthetic peptide of hemagglutinin (HA) corresponding to an antibody binding region of the HA1 subunit (site B: HA1 177-199). The structural requirements for class II-restricted T cell recognition were investigated by determining the proliferative responses of representative CD4+ clones to truncated HA1 peptides and synthetic peptide analogues. Two distinct T cell epitopes were identified and CD4+ clones, specific for either determinant, were sensitive to the same single amino acid substitutions in synthetic peptides at HA1 193 S----N or HA1 198 A----E, that had featured in antigenic drift and abrogated antibody binding to native HA. Competitive inhibition studies, between stimulatory HA1 peptides and non-stimulatory analogue peptides, for antigen presentation to CD4+ clones established that the 193 S----N and 198 A----E substitutions could affect either interaction with the T cell receptor or class II molecule, according to the specificity of the CD4+ clone examined. The structural requirements for class II-restricted T cell recognition of the linear sequence determinants of HA are, therefore, integrally linked to conformation-dependent antibody recognition of the native molecule.