Heat shock proteins were seemingly dominant in immunoscreening strategies, but many researchers now think that they may be involved in pathology and that it would not be a good idea to use them as a subunit vaccine.
Actively secreted protein component of mycobacterium may provide the most protective antigens, and this has drawn increasing interest in the past 5 or 10 years.
Isopentenyl pyrophosphate and other pyrophosphates can stimulate gamma-delta cells, but it is not known whether this immune response can develop a memory.
Mycobacterial lipids can stimulate the so-called CD4-negative, CD8-double-negative T-cell populations, but again, it is not known whether this immune response can develop a memory. It may have a place in immunotherapy strategies.
Pyrophosphates and lipids will probably prove to be important in therapies where the immune response is being induced in the face of disease.
Interest in actively secreted proteins originated with the finding that live vaccines work better than killed vaccines. It has been shown that culture filtrate proteins, the antigens secreted by mycobacteria in vitro, can induce an immune response in mice and guinea pigs and can protect against experimental challenge. Three of these antigens are: MPT-57, which is similar to a heat shock protein; antigen 85 complex (Ag85), probably the hottest candidate (see below); and a so-called less-than-10 kD moiety, which is also secreted by the mycobacterium. Ag85 complex is currently going into clinical trials, but there has been no good comparison of culture filtrate proteins with BCG.
Animal models provide strong evidence that Th-1 CD4-positive cells are important in protective immunity. These cells produce interleukin 2 (IL-2) and interferon gamma (IFN-gamma), both of which activate macrophages to kill intracellular organisms. These findings have been confirmed in the three human models of protective immunity:
People with tuberculosis pleuritis are thought to have an immune response that is controlling the infection, since most of them go on to resolve the disease. Purified lymphocytes from the pleural fluid of these patients produced high levels of IFN-gamma in response to a protein from the cell wall of mycobacterium and to the less-than-10-Kd secreted moiety. This suggests that Th-1 cells are important in this model.
People who are PPD-positive but healthy also appear to have protective immunity, since they have not developed disease. Comparison with TB patients showed that these individuals showed a predominant Th-1 response: elevated levels of IL-2 and IFN-gamma, and depressed levels of IL-4. This suggests that it may be important both to induce Th-1 response and to inhibit Th-2 response.