Person:
Pastor Vargas, Carlos

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First Name
Carlos
Last Name
Pastor Vargas
URI
https://hdl.handle.net/20.500.14352/76123
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Químicas
Department
Bioquímica y Biología Molecular
Area
Bioquímica y Biología Molecular
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1998 - 19992
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Author: Muñoz, Esther × End date: 1999 ×

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    A small domain (6.5 kDa) of bacterial protein G inhibits C3 covalent binding to the Fc region of IgG immune complexes.
    (European Journal of Immunology, 1998) Muñoz, Esther; Vidarte, Luis; Pastor Vargas, Carlos; Casado, Maria Teresa; Vivanco Martínez, Fernando
    Attachment of the complement component C3 to antigen-antibody (Ag-Ab) complexes (immune complexes, IC) is the key molecular event responsible for the elimination of many Ag in the form of Ag-Ab-C3b. The CH1 domain and the Fc region of the Ab, which have previously been involved in the binding of C3b, are also the targets of several bacterial IgG-binding proteins, particularly proteins G and A. Here we describe the ability of a small recombinant protein G domain (B2; 6.5 kDa) to inhibit the covalent binding of C3b to the Fc portion of IgG without affecting the binding to the Fab part. Protein G (B2 domain) produced a remarkable inhibition of covalent binding of C3b to IC formed with rabbit IgG, but none with the F(ab')2 fragment, indicating that B2 interferes with the C3b binding to the Fc region. A weak inhibition was observed with IC formed with mouse IgG2b which preferentially binds B2 domain on the CH1 domain of the Fab. To confirm these data, recombinant single-chain Ab devoid of CH1 domains (scAb), and including the rabbit or human Fc portion (hinge-CH2-CH3), were produced and used to form IC. Protein G-B2 domain inhibited C3b binding to IC formed with scAb of either human or rabbit constant regions, supporting the view of a specific blockade of C3b binding to the Fc region. A similar inhibition of C3b binding was observed using protein A instead of protein G B2 domain and the same set of IC. On the CH1 domain, C3b and B2 bind on opposite faces, and therefore do not interfere with each other in their binding. However, B2 domain bound to the inter-CH2-CH3 region impedes the C3b binding to the Fc. This inhibition clarifies the specificity of C3b for the different regions of IgG and explains how bacterial IgG-binding proteins provide the bacteria with a mechanism of evasion from the opsonizing action of complement and contribute to the virulence. This could be a general mechanism of escape because protein G binds the majority of mammalian Ig.
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    The covalent interaction of C3 with IgG immune complexes
    (Molecular Immunology, 1999) Vivanco Martínez, Fernando; Muñoz, Esther; Vidarte, Luis; Pastor Vargas, Carlos
    Antigens (Ags) are converted into immune complexes (antigen-antibody complexes, IC) as soon as they encounter their specific antibodies (Abs). In fluids containing complement, the process of IC formation and fixation of complement components occur simultaneously. Hence, the formation of Ag-Ab-complement complexes is the normal way of eliminating Ags from a host. C3b-C3b-IgG covalent complexes are immediately formed on interaction of serum C3 with IgG-IC. These C3b-C3b dimers constitute the core for the assembly of C3/C5-convertase on the IC, which are subsequently converted into iC3b-iC3b-IgG by the complement regulators. These complexes are detected on SDS-PAGE by two bands of molecular composition, C3alpha65-C3alpha43 (band A) and C3alpha65-heavy chain of the Ab (band B), which correspond to C3b-C3b and C3b-IgG covalent interaction respectively, and that identify opsonized IC (C3b-IC). C3b can attach to Fab and Fc regions of the Ab molecule with similar efficiency. The presence of multiple C3b binding regions on IgG is considered an advantageous characteristic that facilitates the elimination of Ags in the form of C3b(n)-IC. Ab molecules on the IC recognize the Ag, and also serve as a very good acceptor for C3b binding. In this way, Ags, even if they have no acceptor sites for C3b, can be efficiently processed and removed. When C3 is activated in serum by IC or other activators, secondary C3b-IgG covalent complexes are generated, with bystander monomeric circulating IgG, and thus constitute, physiological products of complement activation. These complexes gain importance when IgG concentration is extremely high as in cases of infusion of intravenous IgG (IVIG) in several pathologies. The covalent attachment of activated complement C3 (C3b, iC3b, C3 d,g) to Ags or IC links innate and adaptative immunity by targeting Ags to different cells of the immune system (follicular dendritic cells, phagocytes, B cells). Hence C3b marks Ags definitively, from the earliest contact with the innate immune system until their complete elimination from the host.