LigB of Leptospira interrogans: Avoiding or exploiting complement?

LigB has turned out to be a versatile surface protein for Leptospira interrogans.  The protein is one of several that the spirochete uses to stick to the extracellular matrix, a critical step in colonizing host tissues.  In addition, LigB's ability to bind fibrinogen may help L. interrogans spread within the host by slowing clot formation.  According to separate studies from the U.S. and Brazil published last year, LigB also helps L. interrogans fend off attack by the host complement system.

What's the evidence that LigB protects Leptospira from complement?  L. interrogans, like many pathogens, survives even when complement is present.  Its resistance to complement is reflected by its ability to survive in human serum, which contains all the complement components necessary to assemble the deadly membrane attack complex within exposed microbial membranes.  On the other hand, the nonpathogen Leptospira biflexa, which lacks the ligB gene, is rapidly killed by human serum.  The U.S. study demonstrated that transformation of L. biflexa with a plasmid expressing LigB allowed the nonpathogen to survive in human serum diluted to 5%, a concentration that easily killed L. biflexa harboring a similar plasmid lacking ligB.

How exactly does LigB protect Leptospira from the onslaught of complement?  A defensive strategy deployed by many pathogens is to seize host complement regulators, which are present to prevent complement activation on host cells.  The Brazilian study showed that LigB grabs several complement regulators that diminish the levels of the key complement proteins C3b and C4b on the bacterial surface.  Two of these regulators are the factor H protein and the C4-binding protein (C4BP).  Both complement regulators break apart the C3 convertases, the enzyme complexes that generate C3b from C3.  Factor H and C4BP also serve as cofactors for the protease factor I, which cleaves C3b and C4b into smaller pieces to prevent their assembly into the C5 convertase.  The C5 convertase is what triggers assembly of the membrane attack complex.  As expected, the investigators found that the breakdown of C3b and C4b by factor I in the presence of its complement regulators was accelerated when LigB was present.

LigB is not the only Leptospira protein that captures complement regulators.  The proteins LenA (originally called LfhA) and LcpA bind factor H and C4BP, respectively.  The importance of factor H in protecting L. interrogans can be seen in the bar graph below.  Survival of L. interrogans was poor in serum lacking factor H.  Addition of factor H to the serum up to the concentration found in blood (500 μg/ml) enhanced survival of the spirochete.

Figure 1 from Castiblanco-Valencia et al. (2012).  Survival of L. interrogans in factor H-depleted serum with 500 μg/ml factor H (FH) is set to 100%.

Surprisingly, the U.S. study revealed that a small segment of LigB grabbed C3b and C4b.  Why would L. interrogans risk capturing complement proteins while simultaneously collecting complement regulators that inactivate those same proteins?  Other pathogens do just fine capturing complement regulators without actively grabbing complement components.  It's possible that C3b and C4b are inactivated more effectively by the complement regulators when all components are bound to LigB.

There's another possibility that should be considered.  What went unmentioned in both papers is that C3b and its cleavage products, which may remain attached to the bacterial surface, are opsonins recognized by phagocytes aiming to grab and engulf microbial intruders.  However, several intracellular pathogens use complement receptors as an entry point to invade macrophages.  L. interrogans has been shown to survive within cultured macrophages and even remained intact within macrophages in a zebrafish model, as I've explained in another post.  Is it possible that L. interrogans uses C3b to grab and invade macrophages?

Featured papers

Castiblanco-Valencia MM, Fraga TR, da Silva LB, Monaris D, Abreu PAE, Strobel S, Jozsi M, Isaac L, & Barbosa AS (2012). Leptospiral immunoglobulin-like proteins interact with human complement regulators factor H, FHL-1, FHR-1, and C4BP. Journal of Infectious Diseases, 205 (6), 995-1004 DOI: 10.1093/infdis/jir875

Choy H (2012). Multiple activities of LigB potentiate virulence of Leptospira interrogans: Inhibition of alternative and classical pathways of complement. PLoS ONE, 7 (7) DOI: 10.1371/journal.pone.0041566

Other helpful papers

Verma A, Hellwage J, Artiushin S, Zipfel PF, Kraiczy P, Timoney JF, and Stevenson B (March 2006).  LfhA, a novel factor H-binding protein of Leptospira interrogans.  Infection and Immunity 74(5):2659-2666.  Link

Barbosa AS, Monaris D, Silva LB, Morais ZM,  Vasconcellos SA,  Cianciarullo AM, Isaac L, and Abreu PAE (July 2010).  Functional characterization of LcpA, a surface-exposed protein of Leptospira spp. that binds the human complement regulator C4BP.  Infection and Immunity 78(7):3207-3216.  DOI: 10.1128/IAI.00279-1010.1128/IAI.00279-10

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