Analysis of complement deposition and processing on Chlamydia trachomatis

Antibodies from chlamydia-infected individuals facilitate phagocytosis via Fc receptors

Non-neutralizing functions of antibodies, including phagocytosis, may play a role in Chlamydia trachomatis (CT) infection, but these functions have not been studied and assays are lacking. We utilized a flow-cytometry-based assay to determine whether serum samples from a well-characterized cohort of CT-infected and naïve control individuals enhanced phagocytosis via Fc-receptor-expressing THP-1 cells, and whether this activity correlated with antibody titers. Fc-receptor-mediated phagocytosis was detected only in CT+ donors. Phagocytosis generally did not correlate well with antibody titer. In addition, we found that complement from both CT+ and negative individuals enhanced phagocytosis of CT into primary neutrophils. These results suggest that anti-CT antibodies can have functions that are not reflected by titer. This method could be used to quantitively measure Fc-receptor-mediated function of anti-CT antibodies or complement activity and could reveal new immune correlates of protection.

Insights into innate immune cell evasion by Chlamydia trachomatis

Chlamydia trachomatis, is a kind of obligate intracellular pathogen. The removal of C. trachomatis relies primarily on specific cellular immunity. It is currently considered that CD4+ Th1 cytokine responses are the major protective immunity against C. trachomatis infection and reinfection rather than CD8+ T cells. The non-specific immunity (innate immunity) also plays an important role in the infection process. To survive inside the cells, the first process that C. trachomatis faces is the innate immune response. As the "sentry" of the body, mast cells attempt to engulf and remove C. trachomatis. Dendritic cells present antigen of C. trachomatis to the "commanders" (T cells) through MHC-I and MHC-II. IFN-γ produced by activated T cells and natural killer cells (NK) further activates macrophages. They form the body's "combat troops" and produce immunity against C. trachomatis in the tissues and blood. In addition, the role of eosinophils, basophils, innate lymphoid cells (ILCs), natural killer T (NKT) cells, γδT cells and B-1 cells should not be underestimated in the infection of C. trachomatis. The protective role of innate immunity is insufficient, and sexually transmitted diseases (STDs) caused by C. trachomatis infections tend to be insidious and recalcitrant. As a consequence, C. trachomatis has developed a unique evasion mechanism that triggers inflammatory immunopathology and acts as a bridge to protective to pathological adaptive immunity. This review focuses on the recent advances in how C. trachomatis evades various innate immune cells, which contributes to vaccine development and our understanding of the pathophysiologic consequences of C. trachomatis infection.

Lack of neutralization of Chlamydia trachomatis infection by high avidity monoclonal antibodies to surface-exposed major outer membrane protein variable domain IV

Chlamydia trachomatis is the leading cause of sexually transmitted diseases causing frequent, long-lasting, and often asymptomatic recurrent infections resulting in severe reproductive complications. C. trachomatis is an intracellular Gram-negative bacterium with a biphasic developmental cycle in which extracellular, infectious elementary bodies (EB) alternate with the intracellular replicating reticulate bodies (RB). The outer membrane of EB consists of a tight disulfide cross-linking protein network. The most essential protein is the 42 kDa major outer membrane protein (MOMP) that contributes to the rigid structural integrity of the outer membrane. MOMP is a transmembrane protein with a β-barrel structure consisting of four variable domains (VD) separated by five constant domains. VDIV possesses surface-exposed species-specific epitopes recognized by the immune system and, therefore, functions as a candidate for vaccine development. To analyze the protective contribution of antibodies for a MOMP vaccine, we investigated the specificity and binding characteristics of two monoclonal antibodies (MAb)224.2 and MAb244.4 directed against C. trachomatis serovar D MOMP. By immunoelectron microscopy, we found that both MAb bind to the surface of C. trachomatis EB. By epitope mapping, we characterized the MOMP epitope as linear consisting of 6 amino acids: 322TIAGAGD328. By ELISA it was shown that both antibodies bind with a higher avidity to the chlamydial surface compared to binding to monomeric MOMP, indicating that the antibodies bind divalently to the surface of C. trachomatis EB. Despite strong binding to the chlamydial surface, the antibodies only partially reduced the infectivity. This may be explained by the observation that even though both MAb covered the EB surface, antibodies could not be regularly detected on EB after the uptake into the host cell.

Beta-lactam induced morphological changes in serum of extended-spectrum beta-lactamase-producing Klebsiella pneumoniae blood isolates

Klebsiella pneumoniae is an opportunistic pathogen, which frequently causes bacteremia. Ceftazidime and meropenem, two important beta-lactam antibiotics for treatment of K. pneumoniae infections, induce morphological changes in bacteria when examined in vitro. Thirty clinical Klebsiella spp. Bacteremia isolates were analyzed for antimicrobial resistance and serum resistance. To determine whether complement influenced the resistance to ceftazidime of extended-spectrum beta-lactamase producing-isolates and sensitivity to meropenem, one serum resistant and one partly serum sensitive isolate were analyzed in normal human serum, heat-inactivated human serum, and growth medium with addition of beta-lactam antibiotics. HA391 was resistant to ceftazidime and had identical minimum inhibitory concentrations for meropenem in normal human serum, heat-inactivated serum and RPMI. In normal human serum, HA233 was inhibited by ceftazidime and had lower inhibitory concentrations of meropenem. Morphological changes induced by serum and beta-lactam antibiotics were analyzed by light- and electron microscopy. Light microscopy showed elongation of bacteria treated with ceftazidime. By electron microscopy membrane attack complexes were observed for HA233 in normal human serum, thereby facilitating beta-lactam antibiotics access to the periplasmic space and the peptidoglycan layer, explaining the increased killing of HA233 by beta-lactam antibiotics. Complement did not enhance beta-lactam killing of HA391, underlining the importance of serum susceptibility.

Complement killing of clinical Klebsiella pneumoniae isolates is serum concentration dependent

Klebsiella pneumoniae is an opportunistic gram-negative pathogen causing serious infections, including sepsis. In plasma, activation of the complement cascades is important for killing bacteria. Thirty clinical Klebsiella spp. blood isolates were analyzed for serum susceptibility in 75% normal human serum (NHS). Twenty-two were serum resistant and eight were serum sensitive, and subsequently tested in 5-75% NHS. Two isolates were killed in 5% and the remaining six in 50%-75% NHS. The two 5% sensitive isolates showed binding of complement (C)4 and C3 in 5% NHS with formation of membrane attack complex (MAC). Inhibition of the classical/lectin mediated pathways (CP/LP) using a C4 specific nanobody, hC4Nb8, led to survival of both isolates in 5% NHS. Using nanobody hC3Nb1, inhibiting the alternative pathway (AP), the isolates were killed in 5% NHS, and amplification of the CP/LP by AP was not necessary for killing. Sole AP killing of these isolates when inhibiting CP/LP with hC4Nb8 was observed in 50% NHS, stressing the concentration dependent functionality of AP. For the less sensitive isolates, killing required activation of CP/LP and AP demonstrated by inhibition with nanobodies. AP inhibition resulted in no C3 deposition on the serum resistant isolate, supporting that AP was the sole activation pathway.

The role of properdin and Factor H in disease

The complement system consists of three pathways (alternative, classical, and lectin) that play a fundamental role in immunity and homeostasis. The multifunctional role of the complement system includes direct lysis of pathogens, tagging pathogens for phagocytosis, promotion of inflammatory responses to control infection, regulation of adaptive cellular immune responses, and removal of apoptotic/dead cells and immune complexes from circulation. A tight regulation of the complement system is essential to avoid unwanted complement-mediated damage to the host. This regulation is ensured by a set of proteins called complement regulatory proteins. Deficiencies or malfunction of these regulatory proteins may lead to pro-thrombotic hematological diseases, renal and ocular diseases, and autoimmune diseases, among others. This review focuses on the importance of two complement regulatory proteins of the alternative pathway, Factor H and properdin, and their role in human diseases with an emphasis on: (a) characterizing the main mechanism of action of Factor H and properdin in regulating the complement system and protecting the host from complement-mediated attack, (b) describing the dysregulation of the alternative pathway as a result of deficiencies, or mutations, in Factor H and properdin, (c) outlining the clinical findings, management and treatment of diseases associated with mutations and deficiencies in Factor H, and (d) defining the unwanted and inadequate functioning of properdin in disease, through a discussion of various experimental research findings utilizing in vitro, mouse and human models.

A Chlamydia trachomatis VD1-MOMP vaccine elicits cross-neutralizing and protective antibodies against C/C-related complex serovars

Ocular and urogenital infections with Chlamydia trachomatis (C.t.) are caused by a range of different serovars. The first C.t. vaccine in clinical development (CTH522/CAF®01) induced neutralizing antibodies directed to the variable domain 4 (VD4) region of major outer membrane protein (MOMP), covering predominantly B and intermediate groups of serovars. The VD1 region of MOMP contains neutralizing B-cell epitopes targeting serovars of the C and C-related complex. Using an immuno-repeat strategy, we extended the VD1 region of SvA and SvJ to include surrounding conserved segments, extVD1^A and extVD1^J, and repeated this region four times. The extVD1^A*4 was most immunogenic with broad cross-surface and neutralizing reactivity against representative members of the C and C-related complex serovars. Importantly, in vitro results for extVD1^A*4 translated into in vivo biological effects, demonstrated by in vivo neutralization of SvA and protection/cross-protection against intravaginal challenge with both SvA and the heterologous SvIa strain.

Nanobodies Provide Insight into the Molecular Mechanisms of the Complement Cascade and Offer New Therapeutic Strategies

The complement system is part of the innate immune response, where it provides immediate protection from infectious agents and plays a fundamental role in homeostasis. Complement dysregulation occurs in several diseases, where the tightly regulated proteolytic cascade turns offensive. Prominent examples are atypical hemolytic uremic syndrome, paroxysmal nocturnal hemoglobinuria and Alzheimer’s disease. Therapeutic intervention targeting complement activation may allow treatment of such debilitating diseases. In this review, we describe a panel of complement targeting nanobodies that allow modulation at different steps of the proteolytic cascade, from the activation of the C1 complex in the classical pathway to formation of the C5 convertase in the terminal pathway. Thorough structural and functional characterization has provided a deep mechanistic understanding of the mode of inhibition for each of the nanobodies. These complement specific nanobodies are novel powerful probes for basic research and offer new opportunities for in vivo complement modulation.