J. G. J. Van De Winkel

Activity of Human IgG and IgA Subclasses in Immune Defense Against Neisseria meningitidis Serogroup B

Both IgG and IgA Abs have been implicated in host defense against bacterial infections, although their relative contributions remain unclear. We generated a unique panel of human chimeric Abs of all human IgG and IgA subclasses with identical V genes against porin A, a major subcapsular protein Ag of Neisseria meningitidis and a vaccine candidate. Chimeric Abs were produced in baby hamster kidney cells, and IgA-producing clones were cotransfected with human J chain and/or human secretory component. Although IgG (isotypes IgG1–3) mediated efficient complement-dependent lysis, IgA was unable to. However, IgA proved equally active to IgG in stimulating polymorphonuclear leukocyte respiratory burst. Remarkably, although porin-specific monomeric, dimeric, and polymeric IgA triggered efficient phagocytosis, secretory IgA did not. These studies reveal unique and nonoverlapping roles for IgG and IgA Abs in defense against meningococcal infections.

Clearance of red cells by monoclonal IgG3 anti-D in vivo is affected by the VF polymorphism of Fc gamma RIIIa (CD16)

Human red cells (RBC) coated with IgG anti‐D are cleared from the circulation to the spleen by macrophages which express IgG receptors (Fcγ R). Polymorphisms of Fcγ RIIa and Fcγ RIIIa affect IgG binding in vitro, and may alter the efficiency of clearance of immune complexes in vivo. In a RBC clearance study, 22 Rh D‐negative subjects were given 100–400u2003µg human monoclonal or polyclonal IgG anti‐D i.m. followed 48 h later by 51Cr‐labelled D+ RBC. The half lives of the infused D+ RBC were determined, together with the coating levels of anti‐D on the D+ RBC. Fcγ RIIA and FcγRIIIA genotyping was performed. Large ranges of phagocytosis and extracellular lysis of RBC in vitro, and of half lives of RBC in vivo, were observed. Clearance of RBC coated with monoclonal IgG3 anti‐D (BRAD‐3) was more rapid in five subjects homozygous for Fcγ RIIIa‐F/F158 than in three subjects expressing the Fcγ RIIIa‐V158 allele (Pu2003=u20030·024). This effect was not observed, however, for those individuals given polyclonal anti‐D. There was also no significant difference in the efficiency of RBC destruction in vitro or of RBC clearance in vivo between the subjects analysed for individual genotypes or alleles or combinations of alleles. In conclusion, the presence of the Fcγ RIIIa‐V158 allele compromised the efficiency of removal of RBC coated with IgG3 anti‐D.

Isotype‐specific cross‐linking of select human FcγR isoforms triggers release of IL‐6

Anti‐CD3 MoAbs arc widely used in T cell activation studies, and are effective in immunosuppressive therapy. We used a panel of mouse (m) anti‐CD3 switch variant MoAbs of five different isotypes to study IL‐6 release from accessory cells. Incubation of human (h) mononuelear cells with anti‐CD3 MoAbs resulted in increased IL‐6 levels with MoAbs of mIgG 1 and mIgG2a isotypes. with no effect of mlgG2b or mIgA. This suggested involvement of IgG Fc receptors (FcγR) in triggering IL‐6 production. To evaluate the role of different FcγR molecules individually we used a panel of hFcγR‐transfeeted mouse fibroblasts, and Jurkat T cells as a model. IL‐6 secretion by CD32 transfectants expressing the hFcγRIIa high‐responder (HR) allelic form was triggered by mIgG1 anti‐CD.3 MoAb. with no effect of four other isotypes. None of the anti‐CD3 MoAbs induced IL‐6 secretion by CD32 transfectants expressing either a variant of this receptor, containing only a single intracellular amino acid (CT‐), the hFcγRIIa low‐responder (LR) allelic form, or hFcγRIIbl. hFcγRI (CD64) transfectants exhibited IL‐6 production after incubation with mlgG2a anti‐CD3 MoAb, and to a lesser extent with mlgG2b. and mlgG I MoAb. Indirect involvement of T cells in triggering IL‐6 secretion could be excluded by experiments in which transfectants were cultured with immobilized anti‐CD3 MoAb. These data indicate that cross‐linking of cither hFcγRI, or hFcγRIIaHR by appropriate anli‐CD3 MoAbs triggers IL‐6 production of accessory cells, and not T cells. This may also take place in vivo during immunosuppressive therapy with anti‐CD3 MoAbs, and related antibody‐mediated immune responses.

Cell-bound IgE and increased expression of Faɛ-receptors on dendritic cells in cutaneous infiltrates of mycosis fungoides

Skin biopsies of 31 non‐atopic patients, 20 with mycosis fungoides, six with psoriasis and five with contact dermatitis, and of five non‐atopic healthy controls were compared for the presence of cell‐bound IgE and vacant IgE binding sites. IgE+ cells were demonstrated in the cutaneous infiltrate of nine (45%) patients with mycosis fungoides, two (33%) with psoriasis and one (20%) with contact dermatitis. Following pre‐incubation of skin sections wilh IgE myeloma protein to saturate vacant IgE‐binding sites, 14 out of 16 patients (88%) with stage I mycosis fungoides, five (83%) patients with psoriasis and one (20%) with contact dermatitis showed an increase in the number of IgE+ cells. While cell‐bound IgE was positively related to serum IgE levels the expression of IgE‐binding sites was not. All IgE+ cells were HLA‐DR+ dendritic cells identified as either macrophages (CD68+, CDI4+) or Langerhans cells (CDl+). Skin biopsies of non‐atopic healthy controls or clinically uninvolved skin in mycosis fungoides had neither any IgE+ cells nor any vacant binding sites. Inhibition studies with IgGl, IgG4 and IgE myeloma proteins as well as with several enzymatic fragments of igE demonstrated that IgE interacled with Fcɛ‐receptors through isotype‐specific structures on the Fcɛ‐fragment. Four anti‐CD23 monoclonal antibodies, however, were unuble to stain vacant Fcɛ‐receptors nor could they block IgE‐binding. We hypothesize that locally‐secreted lymphokines, like IL‐4 or interferon‐γ, induce Fcɛ‐receptors on dendritic cells in the cutaneous infiltrate and that these receptors become occupied in parallel wilh elevated serum IgE levels.

Fcγ Receptors of Myeloid Cells

Fc receptors (FcRs) are cell surface molecules that mediate interactions between cells and immunoglobulins. Phagocytic cells, such as monocytes/macrophages, neutrophils, and eosinophils, need FcRs for the binding, ingestion, and destruction of particles opsonized by antibodies. The FcRs bind the Fc regions of immunoglobulins, thus establishing contact between the cell that carries FcRs on its surface and the antigen bound to the Fab region of the immunoglobulin.