It has long been recognized that parasitic microorganisms possess the ability to infect animals thereby causing disease and often the death of the host. Pathogenic agents have been a leading cause of death throughout history and continue to inflict immense suffering. Though the last hundred years have seen dramatic advances in the prevention and treatment of many infectious diseases, complicated host-parasite interactions still limit the universal effectiveness of therapeutic measures. Difficulties in countering the sophisticated invasive mechanisms displayed by many pathogenic vectors is evidenced by the resurgence of various diseases such as tuberculosis, as well as the appearance of numerous drug resistant strains of bacteria and viruses.
Among those pathogenic agents of major epidemiological concern, intracellular bacteria have proven to be particularly intractable in the face of therapeutic or prophylactic measures. Intracellular bacteria, including the genus Mycobacterium and the genus Legionella, complete all or part of their life cycle within the cells of the infected host organism rather than extracellularly. Around the world, intracellular bacteria are responsible for millions of deaths each year and untold suffering. Tuberculosis, caused by Mycobacterium tuberculosis, is the leading cause of death from infectious disease worldwide, with 10 million new cases and 2.9 million deaths every year. In addition, intracellular bacteria are responsible for millions of cases of leprosy. Other debilitating diseases transmitted by intracellular agents include cutaneous and visceral leishmaniasis, American trypanosomiasis (Chagas disease), listeriosis, toxoplasmosis, histoplasmosis, trachoma, psittacosis, Q-fever, and Legionellosis including Legionnaires' disease. At this time, relatively little can be done to prevent debilitating infections in susceptible individuals exposed to these organisms.
Due to this inability to effectively protect populations from tuberculosis and the inherent human morbidity and mortality caused by tuberculosis, this is one of the most important diseases confronting mankind. More specifically, human pulmonary tuberculosis primarily caused by M. tuberculosis is a major cause of death in developing countries. Capable of surviving inside macrophages and monocytes, M. tuberculosis may produce a chronic intracellular infection. By concealing itself within the cells primarily responsible for the detection of foreign elements and subsequent activation of the immune system, M. tuberculosis is relatively successful in evading the normal defenses of the host organism. These same pathogenic characteristics have heretofore prevented the development of an effective immunotherapeutic agent or vaccine against tubercular infections. At the same time tubercle bacilli are relatively easy to culture and observe under laboratory conditions. Accordingly, M. tuberculosis is particularly well suited for demonstrating the principles and advantages of the present invention.
Those skilled in the art will appreciate that the following exemplary discussion of M. tuberculosis is in no way intended to limit the scope of the present invention to the treatment of M. tuberculosis. Similarly, the teachings herein are not limited in any way to the treatment of tubercular infections. On the contrary, this invention may be used to advantageously provide safe and effective vaccines and immunotherapeutic agents against the immunogenic determinants of any pathogenic agent expressing extracellular products and thereby inhibit the infectious transmission of those organisms.
Currently it is believed that approximately half of the world's population is infected by M. tuberculosis resulting in millions of cases of pulmonary tuberculosis annually. While this disease is a particularly acute health problem in the developing countries of Latin America, Africa, and Asia, it is also becoming more prevalent in the first world. In the United States specific populations are at increased risk, especially urban poor, immunocompromised individuals and immigrants from areas of high disease prevalence. Largely due to the AIDS epidemic the incidence of tuberculosis is presently increasing in developed countries, often in the form of multi-drug resistant M. tuberculosis. 
Recently, tuberculosis resistance to one or more drugs was reported in 36 of the 50 United States. In New York City, one-third of all cases tested in 1991 were resistant to one or more major drugs. Though non-resistant tuberculosis can be cured with a long course of antibiotics, the outlook regarding drug resistant strains is bleak. Patients infected with strains resistant to two or more major antibiotics have a fatality rate of around 50%. Accordingly, a safe and effective vaccine against such varieties of M. tuberculosis is sorely needed.
Initial infections of M. tuberculosis almost always occur through the inhalation of aerosolized particles as the pathogen can remain viable for weeks or months in moist or dry sputum. Although the primary site of the infection is in the lungs, the organism can also cause infection of the bones, spleen, meninges and skin. Depending on the virulence of the particular strain and the resistance of the host, the infection and corresponding damage to the tissue may be minor or extensive. In the case of humans, the initial infection is controlled in the majority of individuals exposed to virulent strains of the bacteria. The development of acquired immunity following the initial challenge reduces bacterial proliferation thereby allowing lesions to heal and leaving the subject largely asymptomatic but possibly contagious.
When M. tuberculosis is not controlled by the infected subject, it often results in the extensive degradation of lung tissue. In susceptible individuals lesions are usually formed in the lung as the tubercle bacilli reproduce within alveolar or pulmonary macrophages. As the organisms multiply, they may spread through the lymphatic system to distal lymph nodes and through the blood stream to the lung apices, bone marrow, kidney and meninges surrounding the brain. Primarily as the result of cell-mediated hypersensitivity responses, characteristic granulomatous lesions or tubercles are produced in proportion to the severity of the infection. These lesions consist of epithelioid cells bordered by monocytes, lymphocytes and fibroblasts. In most instances a lesion or tubercle eventually becomes necrotic and undergoes caseation.
While M. tuberculosis is a significant pathogen, other species of the genus Mycobacterium also cause disease in animals including man and are clearly within the scope of the present invention. For example, M. bovis is closely related to M. tuberculosis and is responsible for tubercular infections in domestic animals such as cattle, pigs, sheep, horses, dogs and cats. Further, M. bovis may infect humans via the intestinal tract, typically from the ingestion of raw milk. The localized intestinal infection eventually spreads to the respiratory tract and is followed shortly by the classic symptoms of tuberculosis. Another important pathogenic vector of the genus Mycobacterium is M. leprae which causes millions of cases of the ancient disease leprosy. Other species of this genus which cause disease in animals and man include M. kansasii, M. avium intracellulare, M. fortuitum, M. marinum, M. chelonei, M. africanum, M. ulcerans, M. microti and M. scrofulaceum. The pathogenic mycobacterial species frequently exhibit a high degree of homology in their respective DNA and corresponding protein sequences and some species, such as M. tuberculosis and M. bovis are highly related.
For obvious practical and moral reasons, initial work in humans to determine the efficacy of experimental compositions with regard to such afflictions is infeasible. Accordingly, in the early development of any drug or vaccine it is standard procedure to employ appropriate animal models for reasons of safety and expense. The success of implementing laboratory animal models is predicated on the understanding that immunodominant epitopes are frequently active in different host species. Thus, an immunogenic determinant in one species, for example a rodent or guinea pig, will generally be immunoreactive in a different species such as in humans. Only after the appropriate animal models are sufficiently developed will clinical trials in humans be carried out to further demonstrate the safety and efficacy of a vaccine in man.
With regard to alveolar or pulmonary infections by M. tuberculosis, the guinea pig model closely resembles the human pathology of the disease in many respects. Accordingly, it is well understood by those skilled in the art that it is appropriate to extrapolate the guinea pig model of this disease to humans and other mammals. As with humans, guinea pigs are susceptible to tubercular infection with low doses of the aerosolized human pathogen M. tuberculosis. Unlike humans where the initial infection is usually controlled, guinea pigs consistently develop disseminated disease upon exposure to the aerosolized pathogen, facilitating subsequent analysis. Further, both guinea pigs and humans display cutaneous delayed-type hypersensitivity reactions characterized by the development of a dense mononuclear cell induration or rigid area at the skin test site. Finally, the characteristic tubercular lesions of humans and guinea pigs exhibit similar morphology including the presence of Langhans giant cells. As guinea pigs are more susceptible to initial infection and progression of the disease than humans, any protection conferred in experiments using this animal model provides a strong indication that the same protective immunity may be generated in man or other less susceptible mammals. Accordingly; for purposes of explanation only and not for purposes of limitation, the present invention will be primarily demonstrated in the exemplary context of guinea pigs as the mammalian host. Those skilled in the art will appreciate that the present invention may be practiced with other mammalian hosts including humans and domesticated animals.
Any animal or human infected with a pathogenic vector and, in particular, an intracellular organism presents a difficult challenge to the host immune system. While many infectious agents may be effectively controlled by the humoral response and corresponding production of protective antibodies, these mechanisms are primarily effective only against those pathogens located in the body's extracellular fluid. In particular, opsonizing antibodies bind to extracellular foreign agents thereby rendering them susceptible to phagocytosis and subsequent intracellular killing. Yet this is not the case for other pathogens. For example, previous studies have indicated that the humoral immune response does not appear to play a significant protective role against infections by intracellular bacteria such as M. tuberculosis. However, the present invention may generate a beneficial humoral response to the target pathogen and, as such, its effectiveness is not limited to any specific component of the stimulated immune response.
More specifically, antibody mediated defenses seemingly do not prevent the initial infection of intracellular pathogens and are ineffectual once the bacteria are sequestered within the cells of the host. As water soluble proteins, antibodies can permeate the extracellular fluid and blood, but have difficulty migrating across the lipid membranes of cells. Further, the production of opsonizing antibodies against bacterial surface structures may actually assist intracellular pathogens in entering the host cell. Accordingly, any effective prophylactic measure against intracellular agents, such as Mycobacterium, should incorporate an aggressive cell-mediated immune response component leading to the rapid proliferation of antigen specific lymphocytes which activate the compromised phagocytes or cytotoxically eliminate them. However, as will be discussed in detail below, inducing a cell-mediated immune response does not equal the induction of protective immunity. Though cell-mediated immunity may be a prerequisite to protective immunity, the production of vaccines in accordance with the teachings of the present invention requires animal based challenge studies.
This cell-mediated immune response generally involves two steps. The initial step, signaling that the cell is infected, is accomplished by special molecules (major histocompatibility or MHC molecules) which deliver pieces of the pathogen to the surface of the cell. These MHC molecules bind to small fragments of bacterial proteins which have been degraded within the infected cell and present them at the surface of the cell. Their presentation to T-cells stimulates the immune system of the host to eliminate the infected host cell or induces the host cell to eradicate any bacteria residing within.
Unlike most infectious bacteria Mycobacterium, including M. tuberculosis, tend to proliferate in vacuoles which are substantially sealed off from the rest of the cell by a membrane. Phagocytes naturally form these protective vacuoles making them particularly susceptible to infection by this class of pathogen. In such vacuoles the bacteria are effectively protected from degradation, making it difficult for the immune system to present integral bacterial components on the surface of infected cells. However, the infected cell's MHC molecules will move to the vacuole and collect any free (released) bacterial products or move to other sites in the host cell to which the foreign extracellular bacterial products have been transported for normal presentation of the products at the cell surface. As previously indicated, the presentation of the foreign bacterial products will provoke the proper response by the host immune system.
The problems intracellular pathogens pose for the immune system also constitute a special challenge to vaccine development. Thus far, the production of an effective vaccine against Mycobacterium infections and, in particular, against M. tuberculosis has eluded most researchers. At the present time the only widely available vaccine against intracellular pathogens is the live attenuated vaccine BCG, an avirulent strain of M. bovis, which is used as a prophylactic measure against the tubercle bacillus. Yet in 1988, extensive World Health Organization studies from India determined that the efficacy of the best BCG vaccines was so slight as to be unmeasurable. Despite this questionable efficacy, BCG vaccine has been extensively employed in high incidence areas of tuberculosis throughout the world. Complicating the matter even further individuals who have been vaccinated with BCG will often develop sensitivity to tuberculin which negates the usefulness of the most common skin test for tuberculosis screening and control.
Another serious problem involving the use of a live, attenuated vaccine such as BCG is the possibility of initiating a life-threatening disease in immunocompromised patients. These vaccines pose a particular risk for persons with depressed cell-mediated immunity because of their diminished capacity to fight a rapidly proliferating induced infection. Such individuals include those weakened by malnourishment and inferior living conditions, organ transplant recipients, and persons infected with HIV. In the case of BCG vaccine, high risk individuals also include those suffering from lung disorders such as emphysema, chronic bronchitis, pneumoconiosis, silicosis or previous tuberculosis. Accordingly, the use of attenuated vaccines is limited in the very population where they have the greatest potential benefit.
The use of live attenuated vaccines may also produce other undesirable side effects. Because live vaccines reproduce in the recipient, they provoke a broader range of antibodies and a less directed cell-mediated immune response than noninfectious vaccines. Often this shotgun approach tends to occlude the immune response directed at the molecular structures most involved in cellular prophylaxis. Moreover, the use of live vaccines with an intact membrane may induce opsonizing antibodies which prepare a foreign body for effective phagocytosis. Thus, upon host exposure to virulent strains of the target organism, the presence of such antibodies could actually enhance the uptake of non-attenuated pathogens into host cells where they can survive and multiply. Further, an attenuated vaccine contains thousands of different molecular species and consequently is more likely to contain a molecular species that is toxic or able to provoke an adverse immune response in the patient. Other problems with live vaccines include virulence reversion, natural spread to contacts, contaminating viruses and viral interference, and difficulty with standardization.
Similarly, noninfectious vaccines, such as killed organisms or conventional second generation subunit vaccines directed at strongly antigenic membrane bound structures, are limited with respect to the inhibition of intracellular bacteria. Like attenuated vaccines, killed bacteria provoke an indiscriminate response which may inhibit the most effective prophylactic determinants. Further, killed vaccines still present large numbers of potentially antigenic structures to the immune system thereby increasing the likelihood of toxic reactions or opsonization by the immune system. Traditional subunit vaccines incorporating membrane bound structures, whether synthesized or purified, can also induce a strong opsonic effect facilitating the entry of the intracellular pathogen into phagocytes in which they multiply. By increasing the rate of bacterial inclusion, killed vaccines directed to intracellular surface antigens may increase the relative virulence of the pathogenic agent. Thus, conventional attenuated or killed vaccines directed against strongly antigenic bacterial surface components may be contraindicated in the case of intracellular pathogens.
In order to circumvent the problems associated with the use of traditional vaccines, developments have been made using extracellular proteins or their immunogenic analogs to stimulate protective immunity against specific intracellular pathogens. For example, this inventor's U.S. Pat. No. 5,108,745, issued Apr. 28, 1992 discloses vaccines and methods of producing protective immunity against Legionella pneumophila and M. tuberculosis as well as other intracellular pathogens. These prior art vaccines are broadly based on extracellular products originally derived from proteinaceous compounds released extracellularly by the pathogenic bacteria into broth culture in vitro and released extracellularly by bacteria within infected host cells in vivo. As disclosed therein, these vaccines are selectively based on the identification of extracellular products or their analogs which stimulate a strong immune response against the target pathogen in a mammalian host.
More specifically, these prior art candidate extracellular proteins were screened by determining their ability to provoke either a strong lymphocyte proliferative response or a cutaneous delayed-type hypersensitivity response in mammals which were immune to the pathogen of interest. Though this disclosed method and associated vaccines avoid many of the drawbacks inherent in the use of traditional vaccines, conflicting immunoresponsive results due to cross-reactivity and host variation may complicate the selection of effective immunizing agents. Thus, while molecular immunogenicity is one indication of an effective vaccine, other factors may complicate its use in eliciting an effective immune response in vivo.
More importantly, it surprisingly was discovered that, particularly with respect to M. tuberculosis, conventional prior art methods for identifying effective protective immunity inducing vaccines were cumbersome and potentially ineffective. For example, SDS-PAGE analysis of bulk M. tuberculosis extracellular protein followed by conventional Western blot techniques aimed at identifying the most immunogenic of these extracellular components produced inconsistent results. Repeated testing failed to identify which extracellular product would produce the strongest immunogenic response and, consistent with prior art thinking, thereby function as the most effective vaccine. Many of the extracellular products of M. tuberculosis are well known in the art, having been identified and, in some cases, sequenced. Further, like any foreign protein, it can be shown that these known compounds induce an immune response. However, nothing in the art directly indicates that any of these known compounds will induce protective immunity as traditionally identified.
Accordingly, it is a principal object of the present invention to provide vaccines or immunotherapeutic agents and methods for their production and use in mounting an effective immune response against infectious bacterial pathogens which do not rely upon traditional vaccine considerations and selection techniques based upon highly specific, strongly immunogenic operability.
It is another object of the present invention to provide vaccines or immunotherapeutic agents and methods for their use to impart acquired immunity in a mammalian host against intracellular pathogens including M. tuberculosis, M. bovis, M. kansasii, M. avium-intracellulare, M. fortuitum, M. chelonei, M. marinum, M. scrofulaceum, M. leprae, M. africanum, M. ulcerans and M. microti. 
It is an additional object of the present invention to provide easily produced vaccines and immunotherapeutic agents exhibiting reduced toxicity relative to killed or attenuated vaccines.