Protection against Mycobacterium ulcerans Lesion Development by Exposure to Aquatic Insect Saliva

Abstract

Background

Buruli ulcer is a severe human skin disease caused by Mycobacterium ulcerans. This disease is primarily diagnosed in West Africa with increasing incidence. Antimycobacterial drug therapy is relatively effective during the preulcerative stage of the disease, but surgical excision of lesions with skin grafting is often the ultimate treatment. The mode of transmission of this Mycobacterium species remains a matter of debate, and relevant interventions to prevent this disease lack (i) the proper understanding of the M. ulcerans life history traits in its natural aquatic ecosystem and (ii) immune signatures that could be correlates of protection. We previously set up a laboratory ecosystem with predatory aquatic insects of the family Naucoridae and laboratory mice and showed that (i) M. ulcerans-carrying aquatic insects can transmit the mycobacterium through bites and (ii) that their salivary glands are the only tissues hosting replicative M. ulcerans. Further investigation in natural settings revealed that 5%–10% of these aquatic insects captured in endemic areas have M. ulcerans–loaded salivary glands. In search of novel epidemiological features we noticed that individuals working close to aquatic environments inhabited by insect predators were less prone to developing Buruli ulcers than their relatives. Thus we set out to investigate whether those individuals might display any immune signatures of exposure to M. ulcerans-free insect predator bites, and whether those could correlate with protection.

Methods and Findings

We took a two-pronged approach in this study, first investigating whether the insect bites are protective in a mouse model, and subsequently looking for possibly protective immune signatures in humans. We found that, in contrast to control BALB/c mice, BALB/c mice exposed to Naucoris aquatic insect bites or sensitized to Naucoris salivary gland homogenates (SGHs) displayed no lesion at the site of inoculation of M. ulcerans coated with Naucoris SGH components. Then using human serum samples collected in a Buruli ulcer–endemic area (in the Republic of Benin, West Africa), we assayed sera collected from either ulcer-free individuals or patients with Buruli ulcers for the titre of IgGs that bind to insect predator SGH, focusing on those molecules otherwise shown to be retained by M. ulcerans colonies. IgG titres were lower in the Buruli ulcer patient group than in the ulcer-free group.

Conclusions

These data will help structure future investigations in Buruli ulcer–endemic areas, providing a rationale for research into human immune signatures of exposure to predatory aquatic insects, with special attention to those insect saliva molecules that bind to M. ulcerans.

Abstract

Saliva from aquatic insects in areas where Buruli ulcer is endemic can protect mice against the disease's characteristic skin lesion and might play a role in natural immunity in humans.

Editors' Summary

Background.

Buruli ulcer disease is a severe skin infection caused by Mycobacterium ulcerans, a bacterium related to those that cause tuberculosis and leprosy. This poorly understood disease affects people living near slow-flowing or standing water in poor rural communities in tropical and subtropical countries. How people become infected with M. ulcerans is unclear but one possibility is that infected aquatic insects transmit it through their bites. The first sign of infection is usually a small painless swelling in the skin. Bacteria inside these swellings produce a toxin that damages nearby soft tissues until eventually the skin sloughs off to leave a large open sore. This usually heals but the resultant scar can limit limb movement. Consequently, 25% of people affected by Buruli ulcers—most of whom are children—are permanently disabled. If the disease is caught early, powerful antibiotics can prevent ulcer formation. But most patients do not seek help until the later stages when the only treatment is to cut out the infection and do a skin graft, a costly and lengthy treatment.

Why Was This Study Done?

There is currently no effective way to prevent Buruli ulcers. To develop an effective preventative strategy, researchers need to determine exactly how the infection is transmitted to people and what makes some individuals resistant to infection. Previous studies have indicated that 5%–10% of some aquatic insect predators that live in areas where Buruli ulcers occur have M. ulcerans in their salivary glands and that aquatic insects carrying M. ulcerans can transmit it to mice through bites. Furthermore, people working close to water inhabited by insect predators are less likely to develop Buruli ulcers than their relatives who do not work near water. In this study, therefore, the researchers investigated whether exposure to noninfected insect saliva provides some protection against M. ulcerans infection.

What Did the Researchers Do and Find?

The researchers let uninfected aquatic insects bite ten mice several times before exposing these mice and ten unbitten mice to M. ulcerans-infected water bugs. Only one pre-bitten mouse developed an M. ulcerans-containing lesion compared with eight control mice. Next, the researchers injected mice with insect salivary gland extracts before challenging them with “naked” M. ulcerans or bacteria coated with salivary gland extract. Most uninjected mice developed lesions when challenged with coated or naked M. ulcerans, as did experimental mice challenged with naked M. ulcerans. However, most experimental mice challenged with coated M. ulcerans remained lesion-free. In both experiments, the blood of the pre-bitten and extract-treated mice (but not the control mice) contained antibodies (immune system proteins that provide protection against infections and foreign proteins) to proteins in insect salivary gland extracts that stick to M. ulcerans. Finally, the researchers measured the blood concentration (the titer) of antibodies that bind insect salivary gland proteins in patients with Buruli ulcer and in healthy people living in the same area. People with high titers of these antibodies, they report, were less likely to have Buruli ulcers than those with low titers.

What Do These Findings Mean?

These findings suggest that exposure to aquatic insect saliva may provide some protection against M. ulcerans lesion development. However, the current results have several limitations. In particular they will only be relevant to human disease if M. ulcerans is normally transmitted by insect bites, and this has not been proven yet. Also, because the human study did not measure the overall immune status of the study participants, the people with Buruli ulcers may have had a general immune deficit rather than simply lacking antibodies against insect salivary gland proteins. However, if the human findings can be repeated and expanded, they suggest that low antibody titers to salivary gland proteins might identify those people who are most susceptible to M ulcerans infections and who would thus benefit most from regular tests for early signs of the disease. Finally, further work on the immune mechanism by which exposure to insect salivary gland proteins protects against M. ulcerans infections may help in the development of vaccines against Buruli ulcer disease.

Additional Information.

Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0040064.

• A related PLoS Medicine Perspective article by Manuel T. Silva and others discusses this study and others on insect-borne parasitic diseases

• World Health Organization has information on Buruli ulcer disease

• US Centers for Disease Control and Prevention has information on Buruli ulcer

• The US Armed Forces Institute of Pathology Web site contains pages on Buruli ulcer

• Leprosy Relief Emmaus Switzerland offers information on Buruli ulcer

• Wikipedia contains pages on Buruli ulcer (note: Wikipedia is an online encyclopedia that anyone can edit)

• PLoS Medicine has a detailed review article on Buruli ulcer by Paul D. R. Johnson and colleagues