**5. Alveolar macrophages in tuberculosis**

following systemic challenge with *M. tb*, it is less rapid in the lung. Slow expression of pro‐ tection in the lung allows mycobacteria to grow and modulate the infection site. Until re‐ cently it has not been clear whether the slow response to aerosol delivery of bacteria resulted from limited availability of antigen or inhibition of antigen-presentation by *M. tb*. Several studies show that the first T cell activation occurs in the draining lymph node (DLN) of the lung 8–10 days following initial challenge. The activation of T cells correlated temporally with the arrival of bacteria and availability of antigen in the DLN, however con‐ ditions for T cell activation were unique to the draining lymph nodes as the presence of an‐ tigen-producing bacteria in the lung and spleen did not result in initial activation of T cells [11, 12]. While delivery of lipopolysaccharide (LPS) to the MTB-infected lung failed to ac‐ celerate T cell priming [11], increasing the bacterial dose did accelerate the response mod‐ estly suggesting that both antigen burden and refractory cells serve to slow the response. So, protective memory cells will not become activated until they see antigen, i.e. more than 8 days post infection. Once T cells become activated they differentiate into effector T cells that migrate to the lung. By day 14 of infection, when activated T cells first arrive in the lung, bacteria are within alveolar macrophages, myeloid DC and neutrophils [11]. T cells can recognize antigen within the mycobacterially-infected lung but the antigen presenta‐ tion is not optimal. It takes time for the protective T cells to reach sufficient numbers to stop bacterial growth. T cells can be divided into two subsets, Th1 and Th2, on the basis of the cytokines they produce. In tuberculosis, Th1 plays a major role in defense against tuber‐ culosis. Th1 cells suppress Th2 cells. CD4 + T cells have unambiguously been identified as the most important lymphocyte subset for mediating protection.CD4 T lymphocytes differ‐ entiate in the peripheral tissues to adopt a variety of fates such as the Th-1 cells, which pro‐ duce interferon (IFN)-*γ* to down-regulate Th2 responses and Th-2 cells, which produce interleukin (IL)-4. CD8 T lymphocytes produce predominantly IFN-*γ*. Though CD4 re‐ sponse is greater than the CD8 response, the latter can provide protection in the absence of CD4 help [13]. During active TB there is a local pulmonary immune response characterized by α/β T cells and strongly enhanced *M. tuberculosis* antigen-specific Th1 responses, with

A wide variety of animal models have been used to test new vaccines and drugs [15]. Mice can harbor high numbers of *M. tb* within lung tissue without showing clinical signs [16]. Mice do not cough nor form cavitary lesions, making them a poor model for transmission studies [17]. Fibrous capsules are not observed histologically, which can affect the validity of antibiotic studies, as *M. tb* would be more easily accessed by drugs in the mouse lung. In addition, because of their short life span, mice are poor models for the study of latent infection. Rat TB also showed similar pathophysiology to murine TB [31]. Guinea pigs develop robust DTH response to mycobacterial antigens and, after infection with *M. tb*, reproduce many of the aspects of human infection, such as caseous and mineralized granulomas, primary and hemato-genous pulmonary lesions, fibrous capsule formation, and dissemination [19],

large amounts of locally secreted IFN-*γ* [14].

130 Tuberculosis - Current Issues in Diagnosis and Management

**4. Animal models of tuberculosis**

When tubercle bacilli reach alveoli, they are phagocytosed by resident alveolar macrophages. Though tubercle bacilli are killed byalveolar macrophages, tubercle bacilli can also kill macrophages through apoptosis. What is the fate of tubercle bacilli once they enter the phagosomes of macrophages? Alveolar macrophages of aerially infected guinea pigs were collected by bronchoalveolar lavage. At 12 days after infection, one out of 10,000 alveolar macrophages of various sizes contained many tubercle bacilli [31]. This indicates that certain alveolar macrophages permit M. tuberculosis to replicate in the phagosomes, although most of tubercle bacilli are killed by activated alveolar macrophages. It will be of great interest to examine the survival mechanism of *M. tuberculosis* at the single-cell level, but we still do not know why macrophages targeted by tubercle bacilli cannot kill the bacilli.

IFN-γ knockout mice were infected with avirulent H37Ra or BCG Pasteur, multinucleated giant cells were recognized in the granulomatous lesions. The lesions also contained tubercle bacilli and consisted of multinucleated cell clusters, being immunopositive with anti-Mac-3 antibody. The alveolar macrophages were transformed into multinucleated ginat cells. We subsequently infected various cytokine-konockout mice with *M. tb*, but no Langerhans' multinucleated giant cells were recognized in the granulomas. Therefore, it seems that formation of multinucleated giant cells requires optimal combinations and concentrations of various cytokines, and the level of IFN-γ, at least, has to be significantly low.
