The genomic sequence of M. leprae has been mapped and completed recently. The genes responsible for its major protein have been cloned and sequenced. One important finding is that M. leprae retains genes for formation of mycobacterial cell wall.The cell wall components of M. leprae stimulate the production of IgM antibodies and cell-mediated immune response, and also moderate the bactericidal activity of macrophages. The cell wall of M. leprae like that of other mycobacteria consists of four layers:
· Peptidoglycan is the innermost layer, which gives the bacte-ria rigidity and shape.
· Lipoarabinomannose-B (LAM-B) is a layer next to peptido-glycan layer.
· A dense palisade of characteristic long-chain fatty acid known as mycolic acid is the third layer attached to LAM-B membrane.
· A layer composed of mycosides is the outermost fourth layer of the cell wall. Phenolic glycolipid (PGL)-1 is the major component of the outermost layer of the cell wall.
M. leprae contains a wide variety of mycobacterial anti-gens. LAM-B is a major antigen of M. leprae. LAM-B is highly immunogenic and induces a high level of serum antibodies. It is distinct from lipoarabinomannose of M. tuberculosis.
PGL-1 is the major antigen component of the outermost layer of the cell wall, which is composed of mycosides. The anti-gen suppresses CMI and protects M. leprae from the action of host cell enzymes. The polysaccharides component of PGL-1 antigen induces development of higher antibodies in patients with leprosy.
Protein antigens: M. leprae consists of a large numberof protein antigens. The major proteinantigens include
(a) 65-kDa heat shock proteins,
(b) 28-kDa protein, and
(c) 18-kDa protein.
1. The 65-kDa heat shock protein induces a humoral anti-body response in the initial phase of M. leprae infection. This antibody response continues to persist during various stages of disease including reactional stage.
2. The 28-kDa antigen protein is the superoxide dismutase of M. leprae. The protein stimulates a higher antibody responsein the lepromatous leprosy than in the tuberculoid leprosy. The antigen also produces even higher antibody response in type 1 reaction.
3. The 18-kDa protein is one of the most important anti-gens, which produces both humoral and cell-mediated responses in a person. All types of leprosy except tubercu-loid leprosy show a high level of antibodies in the serum against 18-kDa antigen.