Involvement of complement in B-cell, T-cell and monocyte/macrophage activation

Complement component C3 plays a critical role in protecting the aging retina in a murine model of age-related macular degeneration

Complement component C3 is the central complement component and a key inflammatory protein activated in age-related macular degeneration (AMD). AMD is associated with genetic variation in complement proteins that results in enhanced activation of C3 through the complement alternative pathway. These include complement factor H (CFH), a negative regulator of C3 activation. Both C3 inhibition and/or CFH augmentation are potential therapeutic strategies in AMD. Herein, we examined retinal integrity in aged (12 months) mice deficient in both factors H and C3 (CFH(-/-).C3(-/-)), CFH alone (CFH(-/-)), or C3 alone (C3(-/-)), and wild-type mice (C57BL/6). Retinal function was assessed by electroretinography, and retinal morphological features were analyzed at light and electron microscope levels. Retinas were also stained for amyloid β (Aβ) deposition, inflammation, and macrophage accumulation. Contrary to expectation, electroretinograms of CFH(-/-).C3(-/-) mice displayed more severely reduced responses than those of other mice. All mutant strains showed significant photoreceptor loss and thickening of Bruch's membrane compared with wild-type C57BL/6, but these changes were greater in CFH(-/-).C3(-/-) mice. CFH(-/-).C3(-/-) mice had significantly more Aβ on Bruch's membrane, fewer macrophages, and high levels of retinal inflammation than the other groups. Our data show that both uncontrolled C3 activation (CFH(-/-)) and complete absence of C3 (CFH(-/-).C3(-/-) and C3(-/-)) negatively affect aged retinas. These findings suggest that strategies that inhibit C3 in AMD may be deleterious.

Immunomodulatory properties of Nocardia lysozyme digest (NLD) in complement normal and C5-deficient mice

The constantly increasing number of substances with adjuvant activity outpaces the elucidation of their mode of action. This problem is of great importance as the immunomodulatory action of an adjuvant is time- and route-dependent, which implies that administration at a different moment or site may result in a reduced immune response. In the present work the possibility to achieve dual effect (stimulatory or inhibitory) is regarded in the light of the complement system. The object of the study is a preparation obtained by lysozyme digestion of Nocardia opaca cell walls (NLD). According to the results, the administration of NLD to mice (i.p. at a daily dose of 0.5 mg kg-1) during 3 days prior to the antigen resulted in an inhibition of serum antibody level against sheep red blood cells (SRBC) and lipopolysaccharide (LPS). At the same time, the preparation stimulated the antibody response to SRBC if it was applied after the antigen. The ability of NLD to ensure protection against experimental infection with Klebsiella pneumoniae was comparatively studied in complement-normal mice (strain ICR) and in C5-deficient mice (strain DBA/2). Firstly, it was established that complement-deficient mice were more resistant to infection than complement-normal. Secondly, the preparation expressed a protective effect in C5-deficient animals; nevertheless the inoculation was done s.c. or i.v. The departure of the infection depended on the rate of opsonization of K. pneumoniae. Under certain conditions NLD can provoke excessive C3 activation, which might aggravate the course of the infection. The preparation augmented the host response to second challenge with K. pneumoniae of complement-normal and C5-deficient mice.

C5 does not play a major role in the immune response of mice to SRBC in vivo

The role of complement component C5 in the immune response of mice to sheep red blood cells (SRBC) was investigated. Congenic C5-sufficient and C5-deficient B10. D2 mice and genetically C5-deficient DBA/2 mice, as such or supplemented with C5-sufficient serum, were used as experimental animals. C5-substitution of the C5-deficient mice resulted in measurable C5 levels for days. The functional half-life of C5 in C5-deficient DBA/2 mice was about 21 h. No significant differences between the IgM-responses of C5-bearing and naive C5-deficient animals were observed. This suggests that C5 does not play a major role in the primary humoral immune response of mice in vivo, although C5 seems to do so in in vitro experiments, even with the same antigen. Antigen-induced C5-production by C5-deficient mice as one of the explanations of the in vitro/in vivo discrepancy could not be confirmed experimentally.

Interaction between C3 and IL-2; inhibition of C3b binding to CR1 by IL-2

We previously reported that C3 has a role in the enhancement of the IL-2 dependent proliferation of helper T cells. Because the IL-2R has a structural homology with the complement proteins, such as CR1 and CR2, we studied the possible ligand crossreactions on CR1 and IL-2-receptor, and the direct interaction between C3 and IL-2. While C3 has an enhancing effect on the IL-2 dependent proliferation of HT-2, a CR1-positive mouse T-cell line, the growth of the CTLL-16 line (CR1-negative) is not affected by C3. It has been proven that neither the insolubilized C3 nor the soluble C3b-like C3 react with the IL-2 binding epitope of the IL-2 receptor. However, using human RBC we have demonstrated that the binding of aggregated C3 to CR1 is inhibited by rIL-2, in a dose-dependent manner. When RBC were incubated with rIL-2 and FITC-labelled Fab-anti-CR1 simultaneously, there was no inhibition in the fluorescence intensity. As detected by ELISA, rIL-2 was bound to the same extent by insolubilized C3, C3b, and C3c, while C3d coat had lower binding capacity. The receptor-binding epitope of IL-2 is intact in the complex of complement proteins and rIL-2, as demonstrated by the binding of DMS1, a monoclonal antibody reacting with the receptor site of IL-2. It is strongly suggested that C3b may play a role in the growth of CR1 positive T cells.

Structural and functional relationships among receptors and regulators of the complement system

The classical and alternative pathway of complement activation are regulated by a series of fluid phase and cell-bound factors, some of which at the same time serve as receptors for fragments of C3 and C4. These molecules are factor H, CR1 (C3b/C4b receptor), CR2 (C3d/EBV receptor), C4BP (C4b binding protein), DAF (decay accelerating factor), MCP (membrane cofactor protein; earlier designated p45/70), CR3 (iC3b receptor or Mac-1) and CR4 (protein 150/95). Due to structural, genetic and functional features these factors are members of one or several newly recognized large families of proteins: (1) molecules with 60 amino acids long repeats (H, CR1, CR2, C4BP, DAF); (2) proteins with 1,2-diacylglycerol membrane anchoring (DAF); (3) proteins with a heterodimer structure and preference for ligands containing the tripeptide arginine-glycine-asparagine (CR3, CR4). Recognizing the above mentioned regulators and receptors of the complement system as belonging to these protein families opens new perspectives for further genetic and functional research of mutual interest to complement and noncomplement scientists.

Expression of CR2/EBV receptors on human thymocytes detected by monoclonal antibodies

The biologic effects of the third component of complement, C3, are mediated via receptors which specifically bind the enzymatic degradation products resulting from the cleavage of C3. One of the products, C3d, has been associated with binding to the second complement receptor CR2 (CD21). This receptor, which is identical to the receptor for Epstein-Barr virus (EBV), has been primarily found on cells of the B lineage, but not on mature T cells or other cells of erythroid or myeloid lineages. In the present investigation, we report the presence of CR2 on human thymocytes. Indirect immunofluorescence analysis employing monoclonal anti-CR2 antibodies revealed a range of thymocyte reactivity from 15% to 63% in thirteen experiments using cells of different donors. Reactivity was always greater with the monoclonal anti-CR2 (CD21) antibody HB-5 than with two other antibodies which recognize distinct epitopes on the CR2 molecule. Two-color immunofluorescence analysis indicated that the brightest of the HB-5-stained thymocytes also reacted with the monoclonal anti-CD1 antibody T6 (immature thymocyte marker) while some of the duller HB-5-staining cells reacted with the monoclonal anti-CD3 antibody Leu-4 (mature thymocyte marker). Immunoprecipitation of CR2 on thymocytes with antibody HB-5 and polyacrylamide gel electrophoretic analysis revealed a protein of 145 kDa molecular mass which is consistent with the size of CR2 found on B lymphocytes. These findings raise several questions regarding the biologic role of CR2-EBV receptor on cells of the T lineage.

Alternative pathway of complement activation by stimulated T lymphocytes. I. Binding of C3 fragments

Human blood lymphocytes cultured for 3 days with concanavalin A (Con A), phytohemagglutinin or pokeweed mitogen, in mixed lymphocyte culture with added interleukin 2 and stimulated by a lymphoblastoid cell line were found to activate and bind C3 molecules when exposed to human serum. The split products of C3 were detected in the supernatants and on the surface of the activated cells. The surface-attached C3 fragment on the Con A blast was identified as C3b by immune adherence i.e. binding of CR1 carrying human erythrocytes. In the Con A-stimulated population the majority of cells that activated and bound C3 were CD3 and Fc gamma receptor (CD16)-positive but complement receptor-negative blasts. In this cell subset both CD4 and CD8-positive cells were detected but their frequency suggested that a proportion of them carried both markers.