Effect of antigen/antibody ratio on macrophage uptake, processing, and presentation to T cells of antigen complexed with polyclonal antibodies

Dye-hydrophobic hapten conjugate/anti-dye antibody complex as immunogen: preparation of hydrophobic hapten-specific monoclonal antibodies

In order to induce the production of antibodies specific for small molecules, it is common to link them to a protein. However, when the small molecule is very hydrophobic it is extremely difficult to prepare such a conjugate. Here, we describe a simple way to obtain an antigenic conjugate under controlled conditions: in a first step a very hydrophobic hapten, cholanic acid, is linked to a dye, basilen blue, in organic solvent; in a second step the cholanic acid-basilen blue conjugate is dissolved in phosphate buffered saline and mixed with rabbit polyclonal anti-basilen blue antibodies previously raised in rabbits against basilen blue-key-hole limpet hemocyanin conjugate. Such a complex, which dissociates very slowly, appears to be a good immunogen in mice. Anti-cholanyl residue monoclonal antibodies were produced and characterized.

Biochemical properties of aeroallergens: contributory factors in allergic sensitization?

Recent studies indicate that the majority of clinically important aeroallergens are biochemically active. A diverse range of properties have been demonstrated but most possess either enzymatic activity (principally hydrolytic), enzyme inhibitory activity, low molecular weight ligand transporting or regulatory properties. In addition, some allergens are glycosylated and/or are structurally similar to proteins which have evolved to function in the respiratory system per se. Little attention has been given to the possibility that the biochemical activity of an allergen or any post-translational modifications contribute to sensitization. In this review, mechanisms with the potential to influence immunogenicity are discussed including interaction with respiratory secretions, epithelial disruption, interactions with immunocompetent cells and receptor mediated endocytosis. Given that many aeroallergens are structurally and functionally similar to a variety of endogenous (e.g. lysosomal enzymes) and exogenous proteins (e.g. microbial enzymes and glycoproteins), particular attention has been directed to the latter. This process represents an important non-adaptive defence mechanism which has evolved to recognize and process such proteins and it is feasible that it plays a similar role in the processing of some allergens entering the respiratory system.

Regulation of B-cell activation and antigen presentation by Fc receptors

Fc receptors are a family of membrane-associated and soluble glycoproteins that mediate a vast array of functions triggered by immune complexes. The structures of murine and human Fc gamma and Fc epsilon receptors have been elucidated and the motifs involved in the activities that they mediate characterized during the past year. B-cell activation and differentiation may be enhanced by different Fc receptor isoforms either through an increased presentation of antigen associated with IgG (Fc gamma RIIb2, Fc gamma RIII, Fc epsilon RII), or the induction of cytokine synthesis by mast cells (Fc epsilon RI, Fc gamma RIII) and natural killer cells (Fc gamma RIII). Conversely, the crosslinking of Fc gamma RIIb1 to membrane Ig inhibits B-cell activation. Soluble forms of Fc receptor also regulate antibody production by enhancing interleukin-4-induced IgE synthesis (Fc epsilon RII) or inhibiting IgG synthesis (Fc gamma R). Different structural motifs are responsible for the different biological activities of each Fc receptor isoform.

Identifying strategies for immune intervention

In recent years the molecular basis of antigen recognition by T cells has been unraveled and the various pathways that control T cell activation and functional specialization have been defined. Consequently, it is now possible to delineate various strategies for intervention with the immune system to design protective vaccines, to induce an effective response to tumor antigens, and to control graft rejection and autoimmune diseases.

Non-epitope-specific suppression of the antibody response to Haemophilus influenzae type b conjugate vaccines by preimmunization with vaccine components

Recently, conjugate vaccines containing Haemophilus influenzae type b capsular polysaccharide (HibCP) coupled to protein carriers were introduced for use in infants and certain adult risk groups. Similar conjugate vaccines against other capsulated bacteria are currently under development for both children and adults. Despite its potential importance, the possible influence of preexisting immunity to the components of such conjugates on the vaccination response in humans has been addressed by few studies. To study this issue, we randomized 82 healthy adult volunteers into six groups and vaccinated them twice, with a 4-week interval between immunizations. Four groups received tetanus toxoid (TT) or diphtheria toxoid (DT) and then HibCP coupled to TT (HibCP-TT) or DT (HibCP-DT). Two groups received HibCP-TT followed by HibCP-DT or vice versa. The total antibody levels to HibCP, TT, and DT and the anti-HibCP immunoglobulin G1 (IgG1) and IgG2 levels were measured before and 4 weeks after the immunizations. For some of the vaccinees, the number of circulating antibody-secreting cells was evaluated 7 days after immunization. Surprisingly, preimmunization with the relevant carrier protein reduced the subsequent increase in the total HibCP antibody levels (P < 0.05), affecting the IgG1 and the IgG2 subclasses equally. Also, the responses to the carrier portions of the conjugates were suppressed, as demonstrated by the lack of significant increases in the antibody levels (P > or = 0.29) and, for HibCP-TT, by reduced numbers of anticarrier antibody-secreting cells (P = 0.009). Similar non-epitope-specific suppression was seen in the groups receiving both conjugates. Thus, preimmunization with one conjugate reduced the subsequent response to the carrier portion of the other conjugate (HibCP-DT and then HibCP-TT, P = 0.00002; HibCP-TT and then HibCP-DT, P = 0.06) as well as to HibCP itself. Possible mechanisms behind this non-epitope-specific suppression and its relevance for vaccine development are discussed.

Role of flanking variable sequences in antigenicity of consensus regions of HIV gp120 for recognition by specific human T helper clones

Human T helper cells can discriminate among strain variants of HIV gp120 because of T cell clones recognizing non-conserved regions, as demonstrated with T cells from HIV-infected individuals and vaccinated volunteers and with primary T cell lines and clones obtained by in vitro immunization. To obtain a better definition of cross-reactions among T cell determinants within HIV gp120 variants, we used a panel of analog peptides within residues 236-251 from the BRU, MN, SF2 and RF strain sequences to induce primary human T cell lines and clones. Different patterns of response were obtained using each of the analog peptides, although they all share the consensus sequence 246-251. Clones recognizing this sequence were generated by priming with the BRU and RF analog peptides, but not with the SF2 analog peptide. SF2 did not induce any 242-245-specific clones, but only T cells recognizing the 236-240 sequence. A preferential response to residues 236-240 was obtained by priming with the BRU and SF2 peptides, but not with the MN and RF peptides. These results suggest that although the analog peptides exhibit a high degree of homology and share a consensus of the C-terminal sequence (246-251), the T cell response to the conserved sequence 246-251 is controlled by flanking sequences. Therefore the presence of a shared sequence per se does not imply in vitro expansion of clones with that fine specificity, even though such clones are available within the naive repertoire and can be triggered by an analog peptide.

T and B cell responses to chimeric proteins containing heterologous T helper epitopes inserted at different positions

The ability of a T helper (Th) epitope to induce help for B cells recognizing different determinants within a multideterminant antigen was investigated. Chimeric fusion proteins, containing inserts of single or multiple copies of the Th epitope ovalbumin 323-339 (ova) at two different positions, were compared with respect to their ability to induce specific antibody production and ova-specific T cell activation. The antibody responses against B cell determinants at the amino and carboxy terminus, respectively was differently influenced by the molecular positioning of the inserted Th determinant. All ova-containing fusion proteins induced antibody production against the B cell determinant at the amino terminal end irrespective of the positioning of ova. In addition, multiple copies of ova in any position led to increased levels of antibody production against this epitope. In contrast, T cell help for antibody production against the determinant at the carboxy terminus was more effective after insertion of multiple copies of ova in a distal than in an adjacent position. Furthermore a fusion protein, containing four copies of ova effectively elicited T cell help for high levels of antibody production against both examined B cell determinants, showing that activated Th cells recognizing a single epitope could simultaneously provide help for distinct sets of B cells specific for widely separated epitopes within a protein. Immunodominant T cell recognition of ova in all chimeric peptides, independently of its position, was demonstrated by lymph node cell (LNC) proliferation of primed BALB/c mice. The level of ova-specific T cell proliferation was similar, irrespective of which chimeric peptide that had been used for priming, and thus did not reveal any differences in T cell priming efficiencies related to the number of ova copies in the fusion proteins. However, when the peptides were presented to a ova-specific T cell line by A20 B lymphoma cells, a close correlation between IL-2 production by the clonal T cells and the number of ova epitopes in the chimeric peptides was observed. Thus, increased cytokine production by ova-specific T cells may be important for the increased level of in vivo antibody production observed in response to multiple copies of ova in the chimeric antigens.

Cytoplasmic domain heterogeneity and functions of IgG Fc receptors in B lymphocytes

B lymphocytes and macrophages express closely related immunoglobulin G (IgG) Fc receptors (Fc gamma RII) that differ only in the structures of their cytoplasmic domains. Because of cell type-specific alternative messenger RNA splicing, B-cell Fc gamma RII contains an insertion of 47 amino acids that participates in determining receptor function in these cells. Transfection of an Fc gamma RII-negative B-cell line with complementary DNA's encoding the two splice products and various receptor mutants indicated that the insertion was responsible for preventing both Fc gamma RII-mediated endocytosis and Fc gamma RII-mediated antigen presentation. The insertion was not required for Fc gamma RII to modulate surface immunoglobulin-triggered B-cell activation. Instead, regulation of activation involved a region of the cytoplasmic domain common to both the lymphocyte and macrophage receptor isoforms. In contrast, the insertion did contribute to the formation of caps in response to receptor cross-linking, consistent with suggestions that the lymphocyte but not macrophage form of the receptor can associate with the detergent-insoluble cytoskeleton.

Intracellular transport of MHC class II molecules

MHC class II molecules associate, during biosynthesis, with peptides derived from endocytosed antigen. Here, Jacques Neefjes and Hidde Ploegh describe the intracellular transport of MHC class II molecules and its relationship to the binding of peptides in endosomal compartments. They discuss alternative routes for the delivery of antigen to sites at which peptides associate with MHC class II molecules and raise the possibility of cell type-specific differences in the handling of MHC class II molecules, and hence in antigen presentation.