BU is a tropical disease receiving far less attention than TB and leprosy, although it is more common in some endemic regions of West Africa. In contrast to these other two mycobacterioses, BU is acquired from the environment following inoculation of M. ulcerans
in the dermis by a mechanism involving aquatic niches and insect vectors although the exact mode of transmission remains unknown 
. BU starts as a painless subcutaneous nodule, oedema or plaque, enlarging over time. As lesions progress, ulcers eventually form that are characterized by an extensive necrosis of subcutaneous tissues accompanied by minimal inflammation 
. The pathology of the disease is closely associated with the production of a lipophilic toxin, mycolactone. This macrocyclic polyketide is highly cytotoxic to a variety of mammalian cells in vitro
, and the injection of mycolactone into the skin of guinea pigs is sufficient to provoke ulcers 
. Although the presence of mycolactone is reflected locally by its damaging effects on infected tissues any investigation of its diffusion outside the ulcerative lesion has been rendered difficult by the lack of a detection tool for this poorly immunogenic compound. In the present study, we have used a radiolabeled mycolactone to show that the toxin diffuses far beyond the sphere of its cytocidal action at the site of inoculation, or at the point of M. ulcerans
Mycolactone is cytotoxic and pro-apoptotic at micromolar concentrations, but in addition non-cytotoxic doses in the nanomolar range can efficiently suppress the functional biology of several types of mononuclear cells. Mycolactone was shown to inhibit the activation-induced production of IL-2 by human lymphocytes, and of TNF by monocytes and macrophages 
. Mycolactone also blocked the capacity of dendritic cells (DCs) to prime cellular responses and to produce chemotactic signals of inflammation 
. Lymphocytes, monocytes, DCs and macrophages compose the mononuclear cell fraction of blood and lymphoid organs. Together, these cell populations contribute to the generation of innate and acquired cellular immune responses, which are critical for protective immunity against mycobacterial infections. The fact that mycolactone targets mononuclear cells in mice infected with M. ulcerans
strongly suggests that these cell subsets are immunosuppressed by the toxin in vivo
The organ distribution of mycolactone revealed a relative tropism of this molecule for the spleen, which is similar to that of other lipophilic immunosuppressive compounds such as rapamycin or FK506 
. However these two drugs are metabolized in the liver, which produces a large array of metabolites 
. Surprisingly, mycolactone was preserved in all the organs (including the liver) that we analyzed by LC-MS/MS, and no metabolite could be identified. Further studies, for example the evaluation of toxin levels in bile, faeces and urine, will be required to determine how infected animals eliminate mycolactone.
In contrast to FK506 and rapamycin, which concentrate primarily in erythrocytes (>90%) and only minimally in lymphocytes (<1%) in circulating blood, we found that mycolactone concentrates in mononuclear cells 
. This is true both for whole blood and for splenocyte cell suspensions, which are richer in lymphocytes. FK506 and rapamycin are structural analogues binding the same intracellular receptor FKBP12, although the resulting complex targets a different molecule. Their sequestration by red blood cells is explained by the high immunophilin levels of erythrocytes. For mycolactone the molecular target and pathway of action of the toxin are still unknown. Our findings, combined with the observation that mycolactone is a potent immunosuppressor of monocytes, macrophages, DCs and lymphocytes suggest that the molecular target of mycolactone may be expressed preferentially by mononuclear cells.
We detected structurally intact mycolactone in PBMCs 6 weeks post infection with M. ulcerans, that is 4 weeks before ulcerative lesions start to develop in this mouse model. This is the first evidence that mycolactone diffuses outside the lesions of an organism infected with M. ulcerans and circulates via peripheral blood. BU is often diagnosed on the basis of clinical findings, because laboratory diagnoses based on smear examination, M. ulcerans cultures, or PCR detection require significant logistics and equipment. Simple and rapid diagnostic field tests for BU are urgently needed for this disease to be treated locally and inexpensively. Our results obtained in the mouse model suggest that assays based on mycolactone detection in circulating blood cells may be considered for diagnostic tests of early disease. We are currently trying to define ways to detect mycolactone directly in the blood cells of BU patients, which may be applicable to diagnosis in field conditions.