Biomedical subjects

A Malbran

Publications and source records attributed to A Malbran.

6 recordsLinked to original sources

PubMed1991

Complement subcomponent C1q stimulates Ig production by human B lymphocytes.

The regulation of Ig production by human B lymphocytes is a complex process involving interactions among B cells, APC, T lymphocytes and soluble factors including activation, growth, and differentiation factors. Components of the complement system, including C3a, C3b, C3d, and C5a, have been shown to influence various stages in this process. In this study, we demonstrate that the C1q subcomponent of complement binds to both small resting and large activated B cells and stimulates immunoglobulin production by Staphylococcus aureus Cowan-activated tonsillar B lymphocytes. This effect is present whether C1q is added to the B cells either at the beginning or near the end of a 7-day culture period and is not associated with enhancement of proliferation. The C1q stimulation of Ig production is, however, associated with increased steady state levels of mRNA for the mu Ig H chain. Furthermore, C1q stimulated IgM production by the human B cell line SKW 6.4, which is capable of secreting IgM in response to B cell differentiation factors (BCDF). SLE is a disorder frequently associated with polyclonal activation of B lymphocytes. We studied the effect of C1q on B cells from two patients with this disorder and one with an SLE-like illness, all selected for the predominance of either IgM or IgG in serum. Spontaneous or BCDF-stimulated Ig secretion was of the isotype predominant in vivo, whereas C1q selectively stimulated B cells to produce the other isotype (IgG vs IgM). Thus, C1q interacts with B lymphocytes in a manner distinct from that of BCDF found in mixed lymphocyte supernatants. C1q may be an important factor influencing the production of Ig by B lymphocytes in normal individuals and in patients with abnormalities of B cell activity.

B-Lymphocytes↗

PubMed1991

Pre-ligation of CR1 enhances IgG-dependent phagocytosis by cultured human monocytes.

Opsonization of the C3b receptor (CR1) on phagocytic cells with C3b enhances both attachment of targets to the cells and subsequent IgG-dependent ingestion of these targets. To explore mechanisms involved in this increased phagocytosis, we adhered cultured human monocytes to surfaces pre-coated with CR1 ligand or control proteins and quantitated ingestion of sheep E opsonized with IgG alone. Three ligands for CR1 resulted in markedly enhanced phagocytosis of targets when compared individually to a panel of non-ligands, as determined by both the proportion of monocytes ingesting targets (percent phagocytosis) and by the number of targets ingested per 100 monocytes (phagocytic index). The ligands included purified C3b, iC3, and Fab fragments of 1B4, a monoclonal anti-CR1, which resulted in a percent phagocytosis of 56.3 (p less than 0.01), 59.0 (p less than 0.01), and 54.4 (p less than 0.02) and a phagocytic index of 281.2 (p less than 0.01), 281.1 (p less than 0.01), and 247.1 (p less than 0.02), respectively. Control proteins including human serum albumin, hemoglobin, Fab fragments of anti-fibronectin, anti-beta 2 microglobulin, and MOPC 21, and Fc fragments of 1B4 and MOPC 21 produced no significant stimulation of phagocytosis, nor did F(ab')2 fragments of monoclonal anti-CR3, M1/70. CR1-specific augmentation of target ingestion was apparent with monocytes cultured in serum-free medium for 1 to 7 days, but was not seen with freshly elutriated cells. Phagocytosis of unopsonized or IgM-coated targets was minimal. These results suggest that the adherent monocytes are primed by CR1 cross-linking for enhanced FcR-mediated phagocytosis even when the CR1 ligand is not present on the targets. This contrasts with the behavior of CR3, and demonstrated functional divergence between these C3 fragment receptors in the phagocytic process.

Antibodies, Monoclonal↗

PubMed1988

Acquired angioedema: observations on the mechanism of action of autoantibodies directed against C1 esterase inhibitor.

We describe a mechanism of action of autoantibodies reactive with the C1 esterase inhibitor (C1EI) molecule found in three patients with acquired angioedema without associated diseases. All of these patients have a circulating C1EI of lower molecular weight than that of normal control subjects. When native C1EI was added to patient, but not control, plasmas, it was cleaved to a lower molecular weight fragment under conditions that allowed contact system activation. In a partially purified system, patient immunoglobulin preparations impaired stable complex formation between plasmin and C1EI and exaggerated the cleavage of the latter to a lower molecular weight fragment. We propose that the autoantibody does not interfere with the cleavage of the bait sequence of the inhibitor but reduces the availability of the reactive center of the molecule in a way that interferes with the irreversible inhibition of target enzymes. In this way unregulated activation of the kinin and complement pathways occurs, leading to disease manifestations via as yet uncharacterized mediators.

Angioedema↗

PubMed1988

CR1-receptor recycling in phorbol ester-activated polymorphonuclear leucocytes.

Complement-receptor type 1, CR1, which recognizes the C3b cleavage fragment of C3, is present on the membranes of human phagocytic cells, but does not mediate phagocytosis or undergo internalization unless activated by one of a variety of stimuli. Among these stimuli low doses of phorbol esters have been shown to induce a consistently increased expression of CR1, despite apparently continuous receptor internalization. We have studied the fate of internalized receptor-ligand complexes in neutrophils activated with low concentrations of phorbol dibutyrate. In our studies, we followed CR1 with either 125I-C3b, the physiologic ligand, or with 125I-Fab fragments of a monoclonal anti-CR1 antibody. We observed rapid internalization of CR1-C3b complexes by PMN treated with 10 ng/ml (1.98 x 10(-8)M) PDBu, consistent in rate and extent with previously reported results using monoclonal antibodies. The fate of the internalized ligand was studied after elution of cell-surface C3b at 0 degrees. Intracellular ligand was externalized in a time- and temperature-dependent fashion, reaching a plateau at 10-15 min. Released C3b was totally TCA precipitable and structurally unaltered, as determined by SDS-PAGE, suggesting that recycling occurs via a prelysosomal predegradative compartment. Loading the cells with chloroquine did not affect this process. A monoclonal anti-CR1 Fab probe behaved in exactly the same manner, suggesting that the recycling of intact ligand-receptor complexes takes place. The possible physiological consequences of this finding are discussed.

Chloroquine↗

PubMed1987

Interactions of monomeric IgG bearing covalently bound C3b with polymorphonuclear leucocytes.

The interactions of monomeric IgG, monomeric C3b, dimeric C3b, and C3b covalently bound to IgG (C3b-IgG) with neutrophils were examined. C3b-IgG heterodimers exhibited an increased affinity of binding to neutrophils, relative to C3b, both at half normal ionic strength (five-fold enhancement) and at normal ionic strength (six-fold enhancement). Binding of monomeric IgG to neutrophils was barely detectable and did not permit direct measurements of affinity in our assay conditions. The increased affinity of C3b-IgG for neutrophils was the result of divalent interactions with CR1, the C3b receptor, and the Fc gamma receptor of the cells, as could be demonstrated by competition studies using unlabelled IgG in the medium or monoclonal antibodies against the neutrophil Fc gamma receptor. This double receptor-ligand interaction allowed C3b-IgG to bind to the cells not only at normal ionic strength, but also in the presence of a 315-fold excess of IgG (near plasma concentration)--conditions that would preclude the binding of C3b or IgG alone. Furthermore, this interaction leads to the internalization of the C3b-IgG complex by the cells. C3b-IgG is internalized with kinetics similar to those found using dimeric C3b, with resting cells demonstrating a slow initial phase, which is accelerated by phorbol ester activation in both cases. Uptake of the homodimeric C3b was greater at 15 min than that of heterodimeric C3b-IgG, but both were significantly greater than that of monomer C3b, which showed no net internalization. The physiological implications of these findings are discussed.

Complement C3b↗

PubMed1987

Phagocytosis of target particles bearing C3b-IgG covalent complexes by human monocytes and polymorphonuclear leucocytes.

Immunoglobulin G (IgG) provides an efficient acceptor site for nascent C3b, and complement activation on the surface of IgG-coated bacteria has been shown to generate significant numbers of C3b-IgG complexes. We have studied the relative efficiency of IgG alone, C3b-IgG complexes, and similar densities of IgG and C3b residues deposited independently, in mediating ingestion of sheep erythrocyte (E) targets by human phagocytes. Human 125I-C3b covalently bound to rabbit anti-Forssman IgG was generated as described elsewhere (Fries et al., 1985). E,EIgMC4b, or EIgMC4b3b (prepared with IgM antibody and purified complement components) were sensitized with radiolabelled anti-Forssman IgG or C3b-IgG heterodimers to generate targets bearing IgG alone, C3b-IgG covalent complexes, or C3b and IgG in equivalent numbers but not bound to each other. Phagocytosis by monocytes and polymorphonuclear leucocytes (PMN) of targets bearing C3b-IgG was markedly enhanced relative to those bearing IgG alone, especially at levels of less than 2000 opsonin residues/target cell. Uptake of C3b-IgG-bearing targets was also significantly more resistant to competitive inhibition by ambient monomeric IgG. Phagocytosis of EIgMC4b + C3b-IgG by monocytes was superior to the uptake of either EAC4b + IgG or EAC4b3b + IgG bearing equivalent amounts of C3b and IgG not in covalent complex (P less than 0.05, n = 10). Similar results were obtained with PMN. Thus, generation of C3b-IgG complexes in vivo may not only promote complement activation and enhance C3b deposition, but also produce a compound opsonic residue which is a more potent promoter of phagocytosis than an equal number of C3b and IgG residues randomly distributed relative to each other.

Animals↗