The Defense Against Infection Deserves a Complement

For most of us, developing a bacterial infection is at a minimum no fun, and usually quite irritating. We don’t really consider what actually goes on inside of our bodies, and as a result, fail to appreciate the vast population of cells and molecules dutifully combating the bacterial invaders. Maybe we ought to reconsider our sentiments towards being sick.

The immune response following each attack of a foreign entity (‘pathogen’) on our bodies is highly complex and actually pretty amazing. Two types of responses may occur – the innate and the adaptive immune responses. The innate response is the first line of defense, and is mitigated by physical barriers like the skin and mucosa lining our digestive and respiratory tracts, as well as by actions like sneezing and coughing. White blood cells (‘leukocytes’) carry out various functions like phagocytosis (engulfing and digesting a large pathogen into small parts) and target cell lysis (killing), which are examples of induced innate responses. If the innate response does not effectively clear our bodies of the pathogen, then certain immune cells (‘lymphocytes’) are subsequently activated to launch a highly specific and intense counterattack involving antibody secretion and precise killing of pathogen.

An important connection between the two types of immune responses is established by something called the complement system. Complement is composed of a collection of various serum (present in bodily fluid) proteins that become sequentially triggered in a closely controlled cascade. The chronological activation of each component protein in the complement system can be thought of as a medley relay race in swimming, where each swimmer swims a different stroke, and the first swimmer must finish his or her part of the race before the next swimmer can begin. However unlike a relay race, where the winners receive an award, complement activation results, for example, in the killing of a pathogen, or in the coating of a pathogen with a particular complement protein (called ‘opsonization’). Opsonization is important in helping leukocytes engulf and digest pathogens more easily, so that the foreign entity may be cleared from circulation and tissue with greater efficiency. Many of the complement proteins are named with the letter C followed by numbers 1-9. Other proteins are named as Factors that have been assigned a single letter (i.e. Factor B, Factor D, etc.). The complement system can be activated by three biochemical pathways: classical, lectin, and alternative. The classical pathway is dependent on the presence of antibody, which means that the adaptive response must be initiated before complement can be activated. However, the lectin and alternative pathways are purely part of innate immunity, and do not involve the interaction with antibody. This review will focus on lectin and alternative complement activation, as an article recently published elucidated a novel role of a protein interestingly involved in both of these pathways (1).

The alternative pathway is initiated when the complement protein C3 spontaneously hydrolyzes (binds a water molecule) and is subsequently acted on by Factor B and Factor D. Eventually, the series of activations of the alternative complement components results in the formation of the membrane attack complex (MAC), creating a pore (small hole) in the membrane of the pathogen and causing it to die. The alternative pathway can also lead to many molecules of C3 becoming deposited on the surface of the pathogen, coating the membrane for opsonization.

The lectin pathway takes advantage of carbohydrates (CHO) displayed on the surface of a pathogen. A protein called MBL (mannose-binding lectin) binds to an array of these carbohydrates, initiating the cascade. Next, the bound MBL associates with a protein complex called MASP (MBL-associated serine protease), and when MASP is activated, additional steps occur to activate each subsequent component one-by-one. Ultimately, this succession of activations can result in the formation of MAC, as well as in the opsonization of the pathogen with C3. Notably, there are three MASPs: MASP-1, MASP-2, and MASP-3. Each MASP has a different role; for example, MASP-2 cleaves the first two complement components in the lectin pathway (2). However, the function of MASP-3 has, until recently, remained unclear.

The latest work by Iwaki and colleagues clarified the mechanism through which MASP-3 is activated, as well as its role in activating the complement system. To accomplish this, the authors used genetically modified Drosophila cells expressing the gene (from mouse DNA) that encodes MASP-3 in order to study the protein at the molecular level. They then extracted the MASP-3 protein (called ‘recombinant MASP-3’, or rMASP-3) from these cells and added it to different types of mouse sera to examine how the protein works. This type of experimentation is in vitro because data is not collected from a living animal or its tissue. First, rMASP-3 was added to wild-type (normal) mouse serum and incubated with either heat-killed bacteria, mannan-agarose gel, or GlcNAc-agarose gel to observe MASP-3 activation. Only when the rMASP-3 protein was incubated with the bacteria, but not with the other substances, did it become activated; MASP-3 activation was also found to be dependent on its binding to MBL, which the authors determined by combining various recombinant forms of the MBL protein in the serum with MASP-3 and bacteria. rMASP-3 activation was further tested in sera from MASP knock out mice (mice that have been engineered to not express one or all of the three MASP proteins). The results showed that rMASP-3 activation was lower in the knock out sera than in the wild-type serum. However, the addition of MASP-1 to the sera significantly increased MASP-3 activation. This suggests that MASP-1 has the ability to activate MASP-3. The authors also determined that MASP-3 is involved in the C3 coating of bacteria, or opsonization as mentioned earlier. Yet, previous research has shown that MASP-3 is not capable of activating particular components in the lectin pathway that are necessary for C3 opsonization (3; 4). Together, these findings indicated that MASP-3 must act through a different pathway in order to mediate the observed C3 opsonization. The authors uncovered that MASP-3 activates Factor B and Factor D in the alternative pathway, resulting in C3 opsonization. This finding establishes a novel mechanism in complement activation.

While this new complement pathway is not likely a major pathway of activation, it may be important in providing support or backup for the alternative complement activation pathway. In the clinical realm, there are various immunodeficiencies where patients lack particular elements of the complement system and suffer from recurrent infections. Thus, understanding all of the possible mechanisms through which complement works helps to distinguish the vulnerability of these patients towards particular types of infection. More broadly, gaining insights on the complexities and overlaps that exist in complement activation could help researchers better understand how people in the general population defend themselves from infections and diseases. For now, this article at the very least highlights the incredible intricacy of the immune response. Hopefully, your next bacterial infection will stimulate some reflection on the dutiful immune cells and proteins defending your body against disease.

Reference:

(1) Iwaki, D., K. Kanno, M. Takahashi, Y. Endo, M. Matsushita, and T. Fujita. 2011. The role of mannose-binding lectin-associated serine protease-3 in activation of the alternative complement pathway. J. Immunol. 187: 3751-3758.

Other References:

(2) Matsushita, M., S. Thiel, J. C. Jensenius, I. Terai, and T. Fujita. 2000. Proteolytic activities of two types of mannose-binding lectin-associated serine protease. J. Immunol. 165: 2637–2642.

(3) Dahl, M. R., S. Thiel, M. Matsushita, T. Fujita, A. C. Willis, T. Christensen, T. Vorup-Jensen, and J. C. Jensenius. 2001. MASP-3 and its association with distinct complexes of the mannan-binding lectin complement activation pathway. Immunity 15: 127–135.

(4) Zundel, S., S. Cseh, M. Lacroix, M. R. Dahl, M. Matsushita, J. P. Andrieu, W. J. Schwaeble, J. C. Jensenius, T. Fujita, G. J. Arlaud, and N. M. Thielens. 2004. Characterization of recombinant mannan-binding lectin-associated serine protease (MASP)-3 suggests an activation mechanism different from that of MASP-1 and MASP-2. J. Immunol. 172: 4342–4350.