Serological tests for leprosy

 

Leprosy Mailing List – February 28,  2013

Ref.:    (LML) Serological tests for leprosy

From:  Dr. Jaison Barreto, Bauru, Brazil

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Dear Editor,

 

Concerning serological tests for leprosy, it is important to clarify many aspects, but two must certainly be reinforced.

The first, and maybe the main, is a immunological one. Leprosy, as many others similar diseases, is caused by an obligate intracellular microorganism. So, in order to detect the antibodies against M.leprae, a minimal amount of antigens must be present in tissue, i.e., inside Schwann cells, the first target of M.leprae. After being infected, this cell must "say" to the immune system that something is "wrong". Unfortunately, Schwann cells are not good APC (Antigen-Presenting-Cells), and there is a barrier between the blood circulation and nervous system. This is maybe why anti-PGL1, a specific antigen, does not work as expected in Indeterminate and Polar Tuberculoid (TT) leprosy, an even in some initial Borderlines (Barreto et al., 2008). I had the opportunity to notice this when the studies with ML-Flow started, almost 10 years ago.

The second problem is the sensitivity and specificity of the test. The presence of circulating antigens (or antibodies), of M.leprae itself does not mean that the individual is sick, particularly not in endemic areas. 

For example, in my PhD thesis, made in the state of Santa Catarina, Brazil, in 2008, I found that more than one third of the household contacts from Lepromatous leprosy patients - these patients were diagnosed and successfully treated with MDT 24 doses (8 years before) - had IgM anti-PGL1 levels higher than 0.15, and no signs or symptoms of active leprosy. Nevertheless, 20% of them also had RLEP-130 (a specific repetitive sequence for M.leprae) positive inside their noses,as detected by PCR assay.

What signifies in itself, the presence of antibodies against M.leprae? Nothing! For example, IgM anti-PGL1 detectable levels can be caused only by the contact of M.leprae with B cells on the nasal mucosa, because these cells are good Antigen-Presenting-Cells, a condition different from Schwann cells.

What must we do to reach to achieve the control of leprosy? Training of health professionals, in the field, like we are doing in Brazil (see www.dahwmt.org.br), and reinforce the importance of follow up of household contacts. Laboratory assays are usually not needed, when a good dermatological and neurological evaluation is done by a skilled health professional.

 

 

Regards, 

 

Dr. Jaison A. Barreto, MD, PhD

Chief of the Leprosy Section

Instituto Lauro de Souza Lima

Bauru - SP - Brazil

 

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