The activation mechanism of human complement system by immune precipitate formed with rabbit IgG antibody

Antibody Targets and Properties for Complement-Fixation Against the Circumsporozoite Protein in Malaria Immunity

The Plasmodium falciparum circumsporozoite protein (CSP) forms the basis of leading subunit malaria vaccine candidates. However, the mechanisms and specific targets of immunity are poorly defined. Recent findings suggest that antibody-mediated complement-fixation and activation play an important role in immunity. Here, we investigated the regions of CSP targeted by functional complement-fixing antibodies and the antibody properties associated with this activity. We quantified IgG, IgM, and functional complement-fixing antibody responses to different regions of CSP among Kenyan adults naturally exposed to malaria (n=102) and using a series of rabbit vaccination studies. Individuals who acquired functional complement-fixing antibodies had higher IgG, IgM and IgG1 and IgG3 to CSP. Acquired complement-fixing antibodies targeted the N-terminal, central-repeat, and C-terminal regions of CSP, and positive responders had greater antibody breadth compared to those who were negative for complement-fixing antibodies (p<0.05). Using rabbit vaccinations as a model, we confirmed that IgG specific to the central-repeat and non-repeat regions of CSP could effectively fix complement. However, vaccination with near full length CSP in rabbits poorly induced antibodies to the N-terminal region compared to naturally-acquired immunity in humans. Poor induction of N-terminal antibodies was also observed in a vaccination study performed in mice. IgG and IgM to all three regions of CSP play a role in mediating complement-fixation, which has important implications for malaria vaccine development.

Human antibodies activate complement against Plasmodium falciparum sporozoites, and are associated with protection against malaria in children

BACKGROUND

Antibodies targeting Plasmodium falciparum sporozoites play a key role in human immunity to malaria. However, antibody mechanisms that neutralize sporozoites are poorly understood. This has been a major constraint in developing highly efficacious vaccines, as we lack strong correlates of protective immunity.

METHODS

We quantified the ability of human antibodies from malaria-exposed populations to interact with human complement, examined the functional effects of complement activity against P. falciparum sporozoites in vitro, and identified targets of functional antibodies. In children and adults from malaria-endemic regions, we determined the acquisition of complement-fixing antibodies to sporozoites and their relationship with antibody isotypes and subclasses. We also investigated associations with protective immunity in a longitudinal cohort of children (n = 206) residing in a malaria-endemic region.

RESULTS

We found that antibodies to the major sporozoite surface antigen, circumsporozoite protein (CSP), were predominately IgG1, IgG3, and IgM, and could interact with complement through recruitment of C1q and activation of the classical pathway. The central repeat region of CSP, included in leading vaccines, was a key target of complement-fixing antibodies. We show that antibodies activate human complement on P. falciparum sporozoites, which consequently inhibited hepatocyte cell traversal that is essential for establishing liver-stage infection, and led to sporozoite death in vitro. The natural acquisition of complement-fixing antibodies in malaria-exposed populations was age-dependent, and was acquired more slowly to sporozoite antigens than to merozoite antigens. In a longitudinal cohort of children, high levels of complement-fixing antibodies were significantly associated with protection against clinical malaria.

CONCLUSIONS

These novel findings point to complement activation by antibodies as an important mechanism of anti-sporozoite human immunity, thereby enabling new strategies for developing highly efficacious malaria vaccines. We also present evidence that complement-fixing antibodies may be a valuable correlate of protective immunity in humans.

Targeting Melanoma Cells with Human High Molecular Weight-Melanoma Associated Antigen-Specific Antibodies Elicited by a Peptide Mimotope: Functional Effects

Human high molecular weight-melanoma associated Ag (HMW-MAA) mimics have been shown to elicit HMW-MAA-specific humoral immune responses that appear to be clinically beneficial. This finding has stimulated interest in characterizing the mechanism(s) underlying the ability of the elicited Abs to exert an anti-tumor effect. To address this question, in the present study, we have generated HMW-MAA-specific Abs by sequentially immunizing rabbits with the peptide P763.74, which mimics the HMW-MAA determinant recognized by mAb 763.74, and with HMW-MAA(+) melanoma cells. HMW-MAA-specific Abs isolated from immunized rabbits mediated cell-dependent cytotoxicity but did not mediate complement-dependent cytotoxicity of HMW-MAA(+) melanoma cells. These Abs also effectively inhibited spreading, migration and Matrigel invasion of HMW-MAA(+) melanoma cells. Besides contributing to our understanding of the role of HMW-MAA in the biology of melanoma cells, these results suggest that both immunological and nonimmunological mechanisms underlie the beneficial clinical effects associated with the induction of HMW-MAA-specific Abs in melanoma patients immunized with a HMW-MAA mimic.

Bactericidal and bacteriolytic activity of serum against gram-negative bacteria

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Solubilization of immune precipitates by six isolated alternative pathway proteins

Immune precipitates were solubilized by the alternative pathway of complement assembled from isolated proteins, i.e., C3, factor B, factor D, properdin, C3b inactivator (C3bINA), and beta 1H. The kinetic curves of solubilization in the isolated system and in EGTA-serum were virtually indistinguishable. No requirement of other factors was apparent. Deletion of C3bINA and beta 1H from the complete mixture caused total consumption of C3 in the fluid phase and resulted in neither C3 binding to the complexes nor solubilization. Thus, the presence of a regulated fluid-phase reaction is essential for efficient fixation of C3 and the consequent solubilization. In addition, properdin plays an essential role in the complement-mediated solubilization in the presence of the two regulators. A large amount of C3 was incorporated into the antigen-antibody lattice. Solubilization of immune complexes started after the binding of one C3 molecule to one antibody molecule in the complexes, and the molar ratio of C3:antibody in the solubilized complexes also is approximately 1.