Native Properdin Binds to Chlamydia pneumoniae and Promotes Complement Activation

Chlamydiae are obligate intracellular Gram-negative bacteria that develop in a host cell within a membrane-bound compartment termed an inclusion. In humans, Chlamydia pneumoniae causes diseases of the respiratory tract, e.g., bronchitis, sinusitis, or pneumonia, with potentially severe sequelae that include atherosclerosis and chronic obstructive pulmonary disease (2, 8, 20). Chlamydia's unique developmental cycle starts when the infectious form of the bacterium, the elementary body (EB), targets the mucosal respiratory epithelium and remains within a nonacidified vacuole known as an inclusion (13). Soon after the infection, the EB differentiates into a noninfectious, but metabolically active, reticulate body (RB), which proliferates by binary fission within the expanding inclusion. The generated progeny differentiates back into EBs that are released upon host cell lysis to infect other cells. During the infection process, Chlamydia does not necessarily remain confined to these primary sites; it also has the propensity to disseminate to extramucosal tissues. For instance, it has been suggested that C. pneumoniae multiplies and survives within macrophages (16, 35, 40) and polymorphonuclear neutrophils (PMN) (42, 50) in order to propagate to the rest of the body and reach endothelial cells. During this journey, from circulating cells to endothelial cells, C. pneumoniae encounters the innate immune system, in which the complement system may play a fundamental role in controlling Chlamydia infection. The complement system is a central component of the innate immune response and is involved in many functions, including recognition, opsonization, phagocytosis, and destruction of foreign cells, as well as generation of chemotactic fragments (C3a and C5a) and activation of adaptive immunity (30, 32, 51) Three pathways of complement activation are known: the classical, lectin, and alternative pathways. Although each uses its own unique mechanism for target versus host discrimination, all pathways result in covalent attachment of C3b to the target and can potentially assemble pores in the bilipid layer of the cell being attacked. The alternative pathway initiates when spontaneously hydrolyzed C3 binds to activating surfaces (i.e., certain bacteria and viruses). Therefore, this pathway does not require a specific antibody response for activation and may play an important role in controlling primary infections with pathogens. Although it is has been described that Chlamydia trachomatis activates the alternative pathway (21, 31), little is known about the effect of this pathway on C. pneumoniae. Moreover, the mechanisms involved in alternative pathway-mediated complement activation or its consequences on infection in C. pneumoniae remain unknown. Properdin is the only positive regulator of the alternative pathway. It is a plasma protein synthesized by monocytes, hepatocytes, mast cells, T cells (45, 46, 49), and shear-stressed endothelial cells (6) and is also a component of the secondary granules in neutrophils (PMN) (53). Patients with properdin deficiency have a higher risk of meningococcal disease than the general population (14), and an association has also recently been found with recurrent otitis media and pneumonia (44). Properdin facilitates alternative pathway complement activation and amplification by extending the half-life of the C3b,Bb convertase (11). The stabilized C3bBb convertase then rapidly cleaves more C3 to C3b, which acts as an opsonin and can reinitiate the pathway in an amplification loop that proceeds on the pathogenic cell. Properdin is composed of cyclic dimers (P2), trimers (P3), and tetramers (P4) of a 53-kDa monomeric subunit (37, 47). Biochemical studies of purified properdin indicate that this protein can form nonphysiological higher-level polymers during events such as long-term storage and freeze-thawing (10, 37). This form, also known as “activated properdin or Pn,” has the abnormal ability to activate complement in solution (37). Recent studies using purified properdin have reported that properdin can act as a pattern recognition molecule and bind directly to surfaces such as dying cells and Neisseria gonorrhoeae in the absence of C3b, serving as a platform for de novo C3bBb assembly (15, 25, 26, 48, 56). Although the data from these studies are consistent with the complement initiation function proposed over 50 years ago (38), we have recently shown that physiological forms of properdin in the absence of artifactual aggregates do bind to late apoptotic and necrotic cells (12, 56) but do not bind to Neisseria spp. (1). Therefore, properdin is likely very selective in its recognition of surfaces. Considering the importance of the recent findings that implicate properdin as a complement initiator on surfaces, we sought to determine whether the physiological forms of properdin (P2, P3, and P4) have the ability to bind to and promote complement activation on the C. pneumoniae surface. Herein, we provide evidence that properdin plays an active role in alternative pathway activation on the C. pneumoniae surface and in controlling infection, suggesting a role for properdin as a specific pattern recognition molecule.