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At least 37 records · Page 2Linked to original sources
PubMed1993

Antibody feedback regulation in MRL/lpr mice.

MRL/Mp-lpr/lpr (MRL/l) mice spontaneously develop autoimmune disease, characterized by glomerulonephritis, polyarteritis and polyarthritis. The lpr mice have defects in the Fas antigen, which plays a role in apoptosis, and it has been suggested that lack of negative selection of autoreactive T cells explains the initiation of the disease. The extremely high amount of autoantibodies may reflect additional immunoregulatory abnormalities. Antibody feedback regulation is an efficient way of up- or downregulating antibody responses. We have for the first time determined whether IgG-mediated suppression as well as IgM-mediated enhancement operates normally in these mice. MRL/l and MRL/Mp(-)+/+ (MRL/n) mice of different ages were therefore immunized with sheep erythrocyte (SRBC)-specific IgG or IgM antibodies followed by SRBC. Control groups received antigen alone. Five days later, the antigen-specific plaque-forming cell response was measured. IgG induced more than 90% suppression in both MRL/l and MRL/n in mice of all ages tested. This degree of suppression is in the same range as for other, normal mouse strains. In contrast, IgM-mediated enhancement was completely absent in 12-week-old MRL/l mice but normal in 8-week-old MRL/l as well as in MRL/n mice of all ages tested. When spleens and lymph nodes were immunohistochemically studied using a mAb specific for complement receptors 1 and 2 (CR1/CR2), an abnormal follicular structure was demonstrated in 12-week-old MRL/l mice. The antibody response of both 8- and 13-week-old MRL/l mice in vivo, after downregulation of these receptors, was inhibited by 85-96%. Thus, the presented data demonstrate that MRL/l mice with overt autoimmune disease are refractory to IgM-mediated enhancement of antigen-specific antibody production. We believe that this abnormal antibody feedback regulation is due to abnormal follicular structure in lymphoid organs of old MRL/l mice, hence the inability to localize and present antigen in a normal way.

Animals↗
PubMed1998

CD21/CD35 in B cell activation.

The stimulus induced via the B cell receptor is a major factor in determination of the fate of naive B cells resulting in activation, elimination or anergy. The strength of B cell signal transduction is dependent not only on the affinity of antigen binding, but by positive signals via the co-receptor CD21/CD19/Tapa-1. Absence of CD21 expression by B cells results in an impaired humoral response to T-dependent antigens which is characterized by a reduction in B cell follicular retention and germinal center survival. The ligand for CD21 is complement C3d which becomes attached to antigen on activation of the complement system. Thus, the complement system of innate immunity is an important regulator of B lymphocytes.

B-Lymphocytes↗
PubMed1999

Role of complement receptors CD21/CD35 in B lymphocyte activation and survival.

In summary, the complement system has evolved an important function in regulation of humoral immunity to T-dependent antigens. Covalent attachment of activated C3 to antigen alters its fate by enhancing uptake on the surface of FDC via CD21/CD35; and by enhancing signal transduction via the B cell coreceptor CD21/CD19/Tapa-1. In the absence of complement receptors CD21/CD35 or C3 ligand, naive B cells bearing low affinity BCR fail to effectively survive within the lymphoid follicle following contact with antigen and death is mediated by a Fas-dependent mechanism. Alternatively, B cells sufficiently activated to initiate a GC reaction fail to survive in the absence of CD21-CD21L interaction.

Animals↗
PubMed2004

Genomic analysis reveals a duplication of eight rather than seven short consensus repeats in primate CR1 and CR1L: evidence for an additional set shared between CR1 and CR2.

We report the discovery of previously unrecognised short consensus repeats (SCRs) within human and chimpanzee CR1 and CR1L. Analysis of available genomic, protein and expression databases suggests that these are actually genomic remnants of SCRs previously reported in other complement control proteins (CCPs). Comparison with the nucleotide motifs of the 11 defined subfamilies of SCRs justifies the designation g-like because of the close similarity to the g subfamily found in CR2 and MCP. To date, we have identified five such SCRs in human and chimpanzee CR1, one in human and chimpanzee CR1L, but none in either rat or mouse Crry in keeping with the number of internal duplications of the long homologous repeat (LHR) found in CR1 and CR1L. In fact, at the genomic level, the ancestral LHR must have contained eight SCRs rather than seven as previously thought. Since g-like SCRs are found immediately downstream of d SCRs, we suggest that there must have been a functional dg set which has been retained by CR2 and MCP but which is degenerate in CR1 or CR1L. Interestingly, dg is also present in the CR2 component of mouse CR1. The degeneration of the g SCR must have occurred prior to the formation of primate CR1L and prior to the duplication events which resulted in primate CR1. In this context, the apparent conservation of g-like SCRs may be surprising and may suggest the existence of mechanisms unrelated to protein coding. These results provide examples of the many processes which have contributed to the evolution of the extensive repertoire of CCPs.

Animals↗
PubMed1993

Evidence for three binding sites for C3 (hemolytically inactive), C3b and C3d on a CR2-positive Burkitt lymphoma-derived cell line (Raji).

The present investigation shows that C3 (hemolytic inactive) as well as C3b and C3d bind Raji, a CR2-positive Burkitt lymphoma-derived cell line. Pretreatment of the cells with OKB-7 inhibited the binding of C3, whereas pretreatment with HB-5 inhibited the binding of C3b. Furthermore, the cells coated either with OKB-7 or HB-5 bound high amounts of C3d. TPA-treated cells showed binding for C3b and weak binding for C3 and C3d. Taken together, the data suggest that Raji cells may express three binding sites for C3, C3b and C3d which can be differently modulated by anti-CR2 MoAbs and TPA.

Antibodies, Monoclonal↗
PubMed1984

Complement C3 receptors: structure and function.

Significant progress has been made during the past few years toward elucidating the structure and function of various C3 receptors. Better understanding of the biochemical basis of the immune adherence phenomenon has been achieved. The availability of polyclonal and monoclonal antibodies to C3 receptors allowed more accurate studies of the cellular distribution of these receptors. Finally, advances in our knowledge of the functional properties of the C3 receptors and their relationship to specific cell surface molecules was facilitated by recognition of a patient deficient in various receptor-mediated functions. All together these developments will soon make it possible to describe in detail the molecular events involved in the modulation of complement receptor coupled cell functions.

Animals↗
PubMed1984

Phenotype characteristics of human B cells studied by Epstein-Barr virus infection. II. C3 receptor switching during human B cell differentiation.

Our experiments showed that two C3 receptors (C3-R) appeared on membrane surfaces in a distinct sequence during the differentiation of human B cells. The experimental results supporting this conclusion were (1) the lymphoblastoid cells established from immature B cells expressed more EACm-R (C3bi and C3d receptors) than EACh-R (C3b receptor), (2) the lymphoblastoid cells originated from mature B cells showed more EACh-R than EACm-R, (3) the above characteristic expression of C3-R was demonstrated on every clone isolated in soft agar from cells in B cell lineage at various differentiation stages, indicating that a cell line consists of homogeneous population in terms of C3-R expression. From these results, it was concluded that C3-R switched from EACm-R to EACh-R during B cell differentiation.

B-Lymphocytes↗
PubMed1989

Proliferation of resting B cells is modulated by CR2 and CR1.

Absence of the third component of complement, C3, is associated with impaired ability to synthesize antibody, particularly in the presence of limiting antigen [1-9]. The mature B lymphocyte bearing the surface immunoglobulin receptor transduces signals for proliferation and differentiation upon binding of specific antigen. This mature B cell also bears two related membrane proteins, CR2 (the C3d/Epstein-Barr virus receptor) (CD35) [15], which can mediate the binding of ligands to which appropriate cleavage fragments of C3 have become attached [16]. It has been suggested that these receptors play a direct role(s) in B cell activation [17-25]. In light of previous in vivo observations we decided to assess the function of CR2 and CR1 in relation to B cell activation through the membrane IgM receptor. Highly purified splenic B cells were prepared. No contaminating T cells or macrophages were detected by flow cytometric analysis and no proliferative activity was present upon PHA or ConA stimulation of the purified cells. The B cells were separated into low (activated), medium (preactivated) and high density (resting) fractions by Percoll gradient density centrifugation [26]. The responses of the B cell subpopulations to various concentrations of anti mu (DA4.4 monoclonal antibody) [27] were examined for proliferation at 72 h and for IgM/IgG production at 7 days. Low density B cells were maximally stimulated and no concentration of anti-mu was effective in enhancing their responses. High density B cells proliferated to anti-mu in a concentration dependent manner. When substimulatory concentrations of anti-mu were employed, concomitant crosslinking of CR2 (with either of 2 distinct monoclonal antibodies HB-5 [28] or OKB7 [17]) resulted in a 45% enhancement of B cell proliferation above that observed by crosslinking of SIgM alone. In these studies, total IgM and IgG did not increase in the absence of T cells or T cell factors, indicating that terminal differentiation did not occur. In contrast, when a monoclonal antibody to CR1(44D) [29] was employed in an identical experiment, B cell proliferation was completely inhibited. Antibodies to CR2 or CR1 either alone or in crosslinked form did not enhance B cell proliferation. Immune complexes may crosslink the B cell surface in a manner analogous to our model when the immunoglobulin receptor and CR2 are simultaneously engaged. This activation signal may be particularly important in eliciting antibody responses when the quantity of specific antigen or the affinity for antigen is low. The marked inhibition of proliferation induced by CR1 suggests an alternate role for this receptor in modulation of B cell responses.

Animals↗
PubMed1992

The evolution of mouse and human complement C3-binding proteins: divergence of form but conservation of function.

Despite the fact that the early components of the mouse and human complement cascades are very similar, there are marked differences between the two species in the structure of C3 receptors and the molecules that control homologous lysis. Here, Michael Holers, Taroh Kinoshita and Hector Molina compare and contrast the mouse and human RCA region products and conclude that the receptor and regulatory roles are conserved despite the structural variation.

Animals↗