Expression of complement receptor 2 (CR2) on HTLV-1-infected lymphocytes and transformed cell lines

Role of complement in immune regulation and its exploitation by virus

Complement is activated during the early phase of viral infection and promotes destruction of virus particles as well as the initiation of inflammatory responses. Recently, complement and complement receptors have been reported to play an important role in the regulation of innate as well as adaptive immune responses during infection. The regulation of host immune responses by complement involves modulation of dendritic cell activity in addition to direct effects on T-cell function. Intriguingly, many viruses encode homologs of complement regulatory molecules or proteins that interact with complement receptors on antigen-presenting cells and lymphocytes. The evolution of viral mechanisms to alter complement function may augment pathogen persistence and limit immune-mediated tissue destruction. These observations suggest that complement may play an important role in both innate and adaptive immune responses to infection as well as virus-mediated modulation of host immunity.

Identification of CD21-binding peptides with phage display and investigation of binding properties of HPMA copolymer-peptide conjugates

Cancer targeting with peptides has become promising with the emergence of combinatorial peptide techniques such as phage display. Using phage display under stringent screening conditions, we selected five distinct peptides that specifically recognized the CD21 receptor, a cell surface marker of malignant B cell lymphoma. Two highly hydrophobic sequences were excluded (RLAYWCFSGLFLLVC and PVAAVSFVPYLVKTY). The binding affinity toward CD21 of the other three selected peptides (RMWPSSTVNLSAGRR, PNLDFSPTCSFRFGC, and GRVPSMFGGHFFFSR) was analyzed with fluorescence quenching. Their dissociation constants were determined to be within the micromolar range. On the basis of the results of phage ELISA, competitive phage ELISA, and fluorescence quenching, the binding sites of the three selected peptides were found to reside within the first four short consensus repeats of CD21 (SCR1-4). The peptide RMWPSSTVNLSAGRR (P1) was bound to the N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer, a potential drug carrier for chemotherapeutic agents, and the surface binding properties of HPMA copolymer-P1 conjugates were investigated. Specific interactions were observed between HPMA copolymer-P1 conjugates and surface-bound receptor. Binding of HPMA copolymer-P1 conjugates was directly related to the amount of surface (MaxiSorp plate) bound receptor, and the binding of the conjugates could be inhibited by the application of a 3-4 orders-of-magnitude excess of free peptide over the peptide concentration in conjugates. The enhanced binding of polymer-bound peptide was ascribed to multivalent interactions between the HPMA copolymer-P1 conjugate and the surface-bound CD21 receptor.

Interactions between worm infections and malaria

Helminths are the most prevalent parasitic infections and malaria is the deadliest parasitic disease. Helminths have been reported to be protective against the severe forms of malaria but they were also possibly linked to increased malaria-incidence and gametocyte carriage. Connecting the dots between observations suggests that statistical regularities throughout the evolution of worms and malaria parasites in the same hosts, may have led to the emergence of non-zero interactions as observed in iterated prisoners dilemma games. Thus by protecting the host, helminths protect themselves and their reproductive potential, but also favor the dissemination and reproduction of Plasmodium falciparum. The proximate causes of this evolutionarily stable strategy might be mediated by IgE and the CD23/NO pathway, the protective role of IL10 in helminth-infected patients, and possibly the hematological consequences of worms. The chronic activation of the CD23/NO pathway might be instrumental in downregulating the expression of cytoadherence receptors thus reducing sequestration of parasitized red blood cells in the deep organs. Mild anemia in helminth-infected patients might favor gametocytogenesis and send attractive cues to the vector. This framework leads to numerous testable hypotheses and could explain certain singularities regarding the double edged role of IgE and NO. Among these hypotheses, there are 2 practical ones: the impact of helminths on malaria vaccine candidates, and the theoretical risk of increasing the severity of malaria after anthelmintics. The capacity for increased IgE responses could thus have been vital in our ancestor's wormy and malarious past. Allergies may be what remains of it in the modern world.

Relationship between reactive nitrogen intermediates and total immunoglobulin E, soluble CD21 and soluble CD23: comparison between cerebral malaria and nonsevere malaria

To search for evidence of a protective role of the CD23/NO pathway against cerebral malaria, concentrations of reactive nitrogen intermediates (RNI) and sCD21, total immunoglobulin (Ig)E and sCD23 were compared between 17 cases of cerebral malaria and 33 controls. The geometric mean of sCD23 concentration was higher among cerebral malaria cases than among controls (optical density 2643/1495, P = 0.01). The ratio between sCD21 and sCD23 was significantly lower in cerebral malaria cases than in controls (0.67 +/- 0.02 versus 0.77 +/- 0.02, respectively, P = 0.009). Multiple linear regression analysis showed that, among cerebral malaria cases, there was a clear correlation between RNI and both IgE (P = 0.007) and sCD21 (P < 0.0001). Among controls, there was a strong negative correlation between RNI and sCD23 concentrations (r = -0.61, P < 0.0001). However, multivariate analysis unmasked the fact that, in controls, there was also a positive correlation between RNI and IgE (P = 0.045). Logistic regression showed that increased RNI concentrations were associated with a cerebral malaria adjusted odds ratio of 1.05 per unit increase [95% confidence interval (CI) 1.006-1.1, P = 0.02] and that an increased ratio between sCD21 and sCD23 was associated with protection from cerebral malaria (adjusted OR = 0.00001 per unit increase (95% CI 0-0.03, P = 0.005). These different immunological profiles suggest that, among controls, the CD23/NO pathway was chronically stimulated whereas, in cerebral malaria, its stimulation was acute, which could explain why some patients developed cerebral malaria and others did not.

Functional properties of soluble CD21

Soluble receptors may display immunoregulatory properties by blocking interactions between ligands and their membrane receptors or by triggering specific biologic responses through interaction with counter part membrane receptors. A natural soluble form of CD21 that is cleaved from lymphocyte membrane CD21 circulates in normal human serum. Soluble CD21 retains the capacity to bind iC3b and CD23, the known ligands of membrane CD21. In a similar fashion to IgE complexes, another ligand of CD23, the soluble CD21 was shown to efficiently trigger CD23-signalling pathways in human monocytes. By inducing release of proinflammatory cytokines and upregulating expression of molecules involved in antigen presentation, soluble CD21 modulates critical monocyte functions that may be relevant to allergic and inflammatory disorders.

An Intronic Silencer Regulates B Lymphocyte Cell- and Stage-Specific Expression of the Human Complement Receptor Type 2 (CR2, CD21) Gene

Human CR2 (CD21) is a B lymphocyte protein whose surface expression is restricted primarily to the mature cell stage during development. To study the transcriptional mechanisms that govern cell- and stage-restricted CR2 expression, we first performed transient transfection analysis using constructs extending from −5 kb to +75 bp (−5 kb/+75) in the CR2 promoter. The promoter was found to be broadly active, with no evidence of cell- or stage-specific reporter gene expression. However, the addition of a 2.5-kb intronic gene segment (containing a DNase I hypersensitive site) to the (−5-kb/+75) construct resulted in appropriate reporter gene expression, defined as the silencing of the (−5-kb/+75) promoter activity only in non-CR2-expressing cells. Interestingly, appropriate reporter gene expression required stable transfection of the constructs in cell lines, suggesting nuclear matrix or chromatin interactions may be important for appropriate CR2 gene expression. Importantly, transgenic mice also required the intronic silencer to generate lymphoid tissue-specific reporter gene expression. Some transgenic founder lines did not demonstrate reporter gene expression, however, indicating that additional transcriptional regulatory elements are present in other regions of the CR2 gene. In summary, these data support the hypothesis that human CR2 expression is regulated primarily by an intronic silencer with lineage- and B cell stage-specific activity.

HIV and human complement: mechanisms of interaction and biological implication

Human complement, although not lytic for HIV-1, interacts with the virus and is closely involved in the infectious process. It enhances infection in the absence of antibody, and turns neutralizing antibodies into agents which increase viral infectivity. In this review M.P. Dierich et al. summarize available information and discuss possible biological implications.