Chemokine receptor homo- or heterodimerization activates distinct signaling pathways

Chemokine receptors of both the CC and CXC families have been demonstrated to undergo a ligand-mediated homodimerization process required for Ca2+ flux and chemotaxis. We show that, in the chemokine response, heterodimerization is also permitted between given receptor pairs, specifically between CCR2 and CCR5. This has functional consequences, as the CCR2 and CCR5 ligands monocyte chemotactic protein-1 (MCP-1) and RANTES (regulated upon activation, normal T cell-expressed and secreted) cooperate to trigger calcium responses at concentrations 10- to 100-fold lower than the threshold for either chemokine alone. Heterodimerization results in recruitment of each receptor-associated signaling complex, but also recruits dissimilar signaling path ways such as G(q/11) association, and delays activation of phosphatidyl inositol 3-kinase. The consequences are a pertussis toxin-resistant Ca2+ flux and trig gering of cell adhesion rather than chemotaxis. These results show the effect of heterodimer formation on increasing the sensitivity and dynamic range of the chemokine response, and may aid in understanding the dynamics of leukocytes at limiting chemokine concentrations in vivo.

Improvement in affinity and HIV-1 neutralization by somatic mutation in the heavy chain first complementarity-determining region of antibodies triggered by HIV-1 infection

We assessed the impact of somatic hypermutation in the framework region 1 (FR1) and complementarity-determining region 1 (CDR1) of three clonally-related heavy chains from the human monovalent antigen-binding fragments Fab S19, S8 and S20 on gp120 binding and HIV-1 neutralization capacity. Nucleotide changes were introduced in the heavy chains to revert single and multiple amino acid residues, and two Fab libraries were constructed with the same light chain to express equivalent amounts of parental and reverted phage Fab. We studied the contribution of each amino acid replacement to antigen binding by calculating the frequency of phage Fab retrieval after competitive library selection on gp120. Whereas mutations in FR1 had no effect on antigen binding, somatic replacements in the CDR1 of the heavy chain (HCDR1) appeared to produce significant changes. In S19 HCDR1, somatic mutation of residue 32 reduced gp120 binding. In Fab S20, the Arg(30) and Asp(31) somatically replaced residues in HCDR1 improved antigen binding. Both of these residues are necessary to increase Fab binding to gp120; reversion of either residue alone results in a decrease in binding. The impact of these two replacements was confirmed by the greater neutralization capacity of S20 compared to the other Fab. Molecular modeling of S20 HCDR1 suggests that Arg(30) and Asp(31) are the main interaction sites for gp120, increasing antibody affinity and promoting the enhanced neutralization ability of S20. These findings are consistent with a gp120-driven process, supporting a role for affinity maturation and intraclonal evolution of HIV-1 neutralizing antibodies.

Natural human antibodies retrieved by phage display libraries from healthy donors: polyreactivity and recognition of human immunodeficiency virus type 1gp120 epitopes

We have characterized the human natural antibody repertoire that contains antibodies recognizing the human immunodeficiency virus type 1 (HIV-1) gp120. A panel of monovalent antigen-binding fragments (Fab) selected from IgM and IgG isotype libraries generated from peripheral blood mononuclear cells (PBMC) of a healthy, HIV-1 noninfected individual was analysed, reflecting that only IgM, but not IgG, Fab were able to recognize HIV-1 gp120. The IgM Fab antibodies were not restricted to any particular heavy chain variable region (VH) germ line gene. However, the recognition of gp120 is associated to polyreactive antibodies and all display low-affinity interaction. This correlates with the absence of any maturation process as somatic mutation or isotype switch as the nucleotide sequence analysis of the variable regions reveals they are expressed near to germline configuration. In addition, none of the antibodies showed any neutralizing activity on HIV-1-infected lymphocytes, reflecting that the natural anti-gp120 repertoire is not sufficient to neutralize HIV infection.

Molecular analysis of HIV-1 gp120 antibody response using isotype IgM and IgG phage display libraries from a long-term non-progressor HIV-1-infected individual

To characterize the variable heavy chain (VH)3 antibody response to HIV-1 gp120, we analyzed a panel of IgM and IgG1 Fab fragments from phage display isotype libraries from a long-term, non-progressor HIV-1-infected individual. The IgM Fab antibodies isolated had low affinity for gp120, were not restricted to a particular VH3 germ-line gene, and consisted mainly of unmutated VH genes. In contrast, IgG Fab fragments were gp120 specific, with high affinity and extensive somatic mutation; all were clonally related and were derived from a single VH3 germ-line gene (DP50). One IgG Fab (S8) has DP50 VH region nucleotide substitutions identical to those of IgM Fab M025 and uses similar DH and JH segments, suggesting that S8 arose from M025 by isotype switching. In addition, somatic mutation in the IgG heavy chain third complementarity-determining region results in a 100-fold affinity increase for gp120, which correlates with a similar increase in neutralization capacity. These results imply that in vivo IgM to IgG isotype switch and affinity maturation may be important for protection and long-term survival in certain HIV-1-infected individuals.

Antibody repertoire against HIV-1 gp120 triggered in nude and normal mice by GM-CSF/gp120 immunization

Granulocyte-macrophage colony stimulating factor (GM-CSF) facilitates the induction of primary immune responses by activating and recruiting antigen-presenting cells (APC), which efficiently present antigen determinants to Th cells. We have derived a functional GM-CSF/gp120 chimeric protein that, following immunization in soluble, adjuvant-independent form in normal mice, triggers highly specific, high affinity anti-gp120 antibodies. In contrast, nude mice respond with mutated, polyreactive, low affinity antibodies that mature further and increase in affinity in T cell-reconstituted nude mice. Anti-gp120 antibody production in nude mice is mediated principally by GM-CSF/gp120-triggered IL-4 production, since neutralizing anti-IL-4 abrogates the in vivo response. The anti-gp120 antibody response in normal, nude and T cell-reconstituted nude mice is encoded at a remarkably high frequency by the VH81X and VH7183 genes, a family used notably during fetal life and, when expressed at the adult stage, associated with autoimmune disease. We conclude that HIV gp120 binds and selects a subpopulation of developing B cells expressing a set of VH genes associated with immunodeficiency and autoimmunity.

Suppression of HIV-1 infection in linomide-treated SCID-hu-PBL mice

BACKGROUND

Proinflammatory cytokine overproduction, as well as synthesis of the inducible form of nitric oxide synthase (iNOS), are known to play a major role in HIV-1-triggered disease. AIDS patients show increased serum tumour necrosis factor (TNF)-alpha and interferon (IFN)-gamma levels, which synergize with HIV-1-produced nitric oxide (NO) to augment viral replication. Linomide has strong immunomodulatory effects in animals and humans, yielding promising clinical benefits in several pathological disorders including septic shock and autoimmune disease, processes largely mediated by overproduction of these cytokines. In peripheral T cells, linomide also prevents apoptosis triggered by a variety of stimuli, including superantigens, dexamethasone and vaccinia virus.

DESIGN AND METHODS

Linomide inhibits production of proinflammatory cytokines such as TNF-alpha, interleukin-1 beta and IFN-gamma, as well as iNOS synthesis. The SCID-hu-PBL mouse model was used to analyse the effect of linomide on HIV-1 infection. T-cell frequency was characterized in reconstituted animals, and the frequency of infected mice and viral load of infected animals were studied.

RESULTS

Linomide promotes an increase in human CD4+ T-cell counts in the peritoneal cavity of HIV-1-infected, linomide-treated mice. Linomide also prevents human TNF-alpha and IFN-gamma production, as well as iNOS expression and affects the viral load, promoting potent suppression of HIV-1 infectivity as detected in peritoneal cavity and spleen.

CONCLUSIONS

The combination of linomide's properties, namely, blockage of proinflammatory cytokine and NO production, as well as prevention of apoptosis, is of paramount interest, making linomide a potential candidate for combating HIV-1 infection or preventing some of its associated pathological manifestations.