Humans constitute the only known reservoir of M. leprae
, except in those select areas with zoonotic leprosy in armadillo populations 
. It is generally assumed that all diseased individuals must have contracted leprosy directly or indirectly from another infected person. However, the inability to recognize subclinical or latent infections in association with the long incubation time have hampered our knowledge about the mode and source of M. leprae
transmission and the risk factors associated with disease manifestation among infected individuals. In the previous 
and present study, we showed that a set of M. leprae
MHC class I and class II-restricted peptides can specifically identify individuals exposed to M. leprae
infection and with active disease. The set of M. leprae
-specific peptides clearly differentiated individuals from an endemic area (Fortaleza) from those living in a non-endemic site for leprosy in Brazil, but with endemic tuberculosis (Porto Alegre).
Our previous 
and present studies were conducted in distinct endemic sites of Brazil (Rio de Janeiro and Fortaleza), suggesting that despite differences in genetic background, a very similar combination of peptides could efficiently discriminate between exposed and unexposed individuals in other endemic countries. We propose as a major application for this test its use as an epidemiological tool by National Leprosy Control Programs, to define the magnitude of the infected population and consequently of transmission in an endemic area for leprosy. Currently a calibration curve of leprosy new case detection rate versus IFN-γ levels is under construction by evaluating sites with increasing annual leprosy new case detection rates. In addition these peptides are also being analyzed in other leprosy endemic areas in Ethiopia and Asia in order to estimate their use on a worldwide basis.
As a positive response to PGL-I, a specific marker of M. leprae is an indicator of bacillary load, the combination of these two observations pointed to a role for M. leprae or M. leprae components in negatively modulating IFN-γ production in infected individuals, and perhaps contributing to the evolution from infection to active disease in M. leprae-exposed individuals.
These observations were combined to elaborate a model relating IFN-γ production with the initially asymptomatic M. leprae infection, and as the infection progresses to disease, a down regulation of M. leprae-specific IFN-γ production (). The relative IFN-γ levels of the different groups were derived from the median values shown in . This observation suggested a new role for the continuous exposure to live M. leprae seen in contacts of multibacillary leprosy patients, not only allowing M. leprae infection of the HCMB, but also negatively modulating the immune response to this bacillus, available from an exogenous source in the HCMB, or an endogenous source for the PB and MB patients. The distribution of groups in also raised the possibility that if we want to evaluate immune response in the asymptomatic phase of M. leprae infection, patients are not the best option for positive responses. Of note, a similar graph was generated based on the production of IFN-γ in response to the whole bacteria (; except for groups living in sites with low/medium prevalence rates in which a higher background is seen with whole bacteria probably due to cross reactivity), indicating that the immune response to the peptides follows a similar trend as to the whole bacterium.
A model for modulation of IFN-γ production during asymptomatic M. leprae infection and active disease.
The proposed model associated the NECBrazil and EClow groups as the individuals with no or reduced exposure to M. leprae infection. A second group can include healthy controls from high endemicity areas (EChigh) and contacts of PB patients (HCPB). The HCMB group already had intermediate values for IFN-γ, and the patients were at the lower end among the groups of infected individuals. So we tested by ANN the possibility of discriminating infected individuals from patients. The group combining EChigh and HCBP was evaluated against MB and PB patients and the decline in IFN-γ levels allowed the correct discrimination of patients in 84.21% of the cases.
A very interesting observation among the groups of non-contact healthy individuals was the correlation between the levels of IFN-γ produced by each group and the degree of exposure to M. leprae
. While the IFN-γ levels were (almost) absent in individuals living in areas with low/medium prevalence rates (NECBrazil
groups), in residents of high-prevalence neighborhoods of Fortaleza (EChigh
group), levels were comparable to those seen in household contacts of leprosy patients. These data indicate that in areas with high prevalence rates, the exposure to M. leprae
is independent of a previous history of contact with leprosy patients. These results are in agreement with previous studies indicating widespread M. leprae
nasal carriage as determined by PCR among the general population in an area in which leprosy is endemic 
. Moreover, they support the view that prolonged intimate contact with a leprosy patient is not required for transmission as has been shown in studies on medical personnel 
, and may explain why a good proportion of incident cases arise among individuals with no previous history of contact with leprosy 
. Our data also support the general view that M. leprae
is highly infectious but poorly pathogenic and that most individuals exposed to M.leprae
present a subclinical infection and develop a protective immune response against this bacillus.
Although close contact is not critical for infection, it seems to play a key role in leprosy manifestation. The critical role of IFN-γ in controlling M. leprae
infection was first described by Nogueira et al. 
who demonstrated that lepromatous leprosy and borderline lepromatous patients, in deep contrast to tuberculoid patients, failed to release this cytokine in response to specific antigen. In our study, as shown in and , the peak of IFN-γ median production was observed in household contacts of paucibacillary patients (HCPB). Starting from this group, the IFN-γ levels in response to M. leprae
or M. leprae
-specific peptides is progressively reduced when groups of increasing levels of exposure to M. leprae
are compared (HCMB), and are further diminished in leprosy patients. The relatively lower response of contacts of multibacillary patients in comparison to contacts of paucibacillary patients suggests that the evolution of latent infection to active disease is associated with progressive reduction in pathogen-specific IFN-γ production, perhaps in parallel with increase in bacillary load. This down modulation of effector response to M. leprae
(Ex. IFN-γ levels) in consequence of long-term and constant stimulation of the immune system by the exogenous bacillus released by the index case is a possible explanation for the well-known increased risk of household contacts of multibacillary leprosy patients to develop leprosy 
. Indeed, the observation that “super exposure” to M. leprae
can lead to a decrease in host resistance was first described in 1973 
. In this study, the authors used a lymphocyte transformation test to show that contacts of lepromatous patients with active disease displayed lower in vitro responses to M. leprae
when compared with contacts of lepromatous patients treated for more than six months.
Interestingly, HCPB with positive serology to PGL-I produced significantly lower levels of IFN-γ in response to M. leprae
-specific peptides when compared to PGL-I negative individuals. No similar influence of levels of anti-PGL-I antibody was observed among HCMB, may be because in this case the IFN-γ levels were already down modulated due to the high bacterial exposure. The level of anti-PGL-1 antibody has been considered as a reliable marker of bacterial load in leprosy patients; anti-PGL-1 levels are associated with the disease spectrum and decline upon treatment (for a review see Oskam et al.
, 2003). Moreover, a higher risk of developing leprosy has been found among household contacts seropositive to anti-PGL1 
. Thus, PGL-I serology in association with IFN-γ levels in response to the peptides may constitute a robust test for detecting infected individuals with higher bacterial loads and more risk of developing leprosy. The combination of tests for PGL-I specific antibodies and IFN-γ in response to M. leprae
-specific peptides may require a follow-up study for evaluating patterns of response associated with evolution to active disease or protection. This is currently under investigation at various endemic sites (Geluk et al.
, for IDEAL consortium).
The studies on the immune response and models of leprosy pathogenesis have been concentrated in active cases that constitute less than 1% of the infected population. Some observations point to the inhibition of dendritic cell maturation and the low frequency of DC s in the lepromatous leprosy lesions as examples of the negative modulation of M. leprae
-specific immune response in leprosy 
. But, as seen in our observations in contacts of MB patients and endemic controls of hyperendemic areas, prior to active disease, PBMC from M. leprae
-exposed individuals respond to M. leprae
-specific stimuli with high IFN-γ levels. So, at least initially, priming to M. leprae
and differentiation of a Th1 T cell response takes place. But, failure of DCs in inducing priming and Th1 differentiation of M. leprae
can be a possible mechanism for lower levels of response in HCMB and patients, especially if components of M. leprae
such as PGL-I are the culprits 
. In the course of the chronic stimulation of Th1 and other T cell subsets seen in human and murine diseases such as visceral leishmaniasis, there is induction of IL-10 production by the IFN-γ producing T cell and subsequent down regulation of Th1 differentiation 
. This is a mechanism that could be relevant for the modulation of response in M. leprae
latent infection, with potential relevance for the development of a prognostic test and/or vaccines for leprosy.