Tuberculosis is caused by the bacteria Mycobacterium tuberculosis, and is characterized by an infection of the bronchi (a part of the lungs) in which macrophages (a type of white blood cell) are unable to digest the bacteria. As a result, macrophages release cytokines, small proteins, that attract T lymphocytes, which kill cells by releasing "killer proteins" and create a granuloma (a collection of immune cells surrounding infected tissue) (Knechel 2009). Tuberculosis poses a threat to the health of the world: in 2007, there were approximately 1.77 million deaths from tuberculosis—the second highest death rate of any infectious disease (Glaziou et al. 2009). It is also a highly mutative pathogen: cases of extreme-multidrug resistant tuberculosis are common, and the treatment responses are usually poor, while the mortality rates are high (Telzak et al 1995).
A recent study by Beham et al. published in PLoS pathogens examined how T-cell receptors (TCRs), which recognize a protein-complex on other cells, on macrophages affect the formation of tuberculous granulomas. To begin the study, the researchers had to establish that monocytes, precursors to macrophages, and macrophages do in fact express T-cell receptors. To do this the researchers used antibodies, proteins that target specific molecules, targeted to TCRα/TCRβ and MHC-II (this is the protein complex recognized by TCRs). From this experiment, they were able to establish that approximately 5% of monocytes expressed TCRαβ (a type of TCR). Although the presence of TCRs in monocytes appeared to be promising, the researchers continued by testing monocyte-derived macrophages from three donors. These monocytes were activated using IFNγ and IL-4, both cytokines that are responsible for initiating inflammation. The researchers found that 5% of the naïve macrophages expressed TCRs, while 9% IL-4 of and 11% of IFNγ activated macrophages expressed TCRs.
The researchers then tested to see how different the TCRs on the activated macrophages were. Activated macrophages are cells that have come in contact with the protein (or molecule) that their TCR recognizes as well as cytokines. They found that on macrophages activated by IFNγ the TCRs tended to be more diverse in the V region (a part of the TCR) than IL-4 activated macrophages. This becomes important when an infection is recognized in the human body. In some cases, a Th1 response (inflammatory response) is initiated, and cytokines such as IFNγ and IL-4 are released. When this happens some macrophages begin to express TCRs. Depending on the makeup of the extracellular environment, the macrophages will have a greater (or lesser) diversity of TCRs. Beham et al. then went on to test whether or not the TCR inhibited macrophage activity. By using “baits” which were beads covered in anti-TCR antibodies (targeted for TCRs), the researchers found that even after 15 minutes there was an increase in macrophage response (the increase in response was not significant until one hour however). The researchers then tested this system in vivo using Mycobacterium bovis Bacille-Calmette-Guérin (M bovis BCG) and found a response similar to the beads.
Beham et al next decided to see how the presence of mycobacteria in the macrophage influenced TCR expression and granuloma formation. Using a labeled M. bovis BCG (labeled means it can be tracked in an organism) they found that macrophages infected with the bacteria had a 400% increase in the expression of TCRs. This in turn leads to more effective clustering and granuloma formation. Interestingly, the researchers also found that a specific region of the TCR, known as the TCRvβ (T-cell receptor variable beta, another region of a TCR) region was highly conserved among the macrophages. This is important because it points to a possibility that the body has a specialized mechanism for fighting infections such as tuberculosis. Further investigation of this conserved region could lead to a better understanding of granuloma formation, and more importantly, why granulomas break apart, allowing latent infections to become active. As well, these macrophages with TCRs were found in 10 of 13 patients with pulmonary tuberculosis.
To understand why the macrophages expressed TCRs, Beham et al tested the effects of tumor necrosis factor (TNF), a cytokine associated with inflammatory and macrophage responses. They found that when blocking TNF, expression of TCRs on macrophages was suppressed within 2 hours. This suppression could be reversed however with exposure to TNF, and new TCRs would be expressed within 24 hours. This then indicates that TNF is necessary for TCR expression on macrophages. TNF blockade, a system by which TNF is blocked, caused the degradation of CD3ζ, a necessary protein for stabilization of the TCR. Understanding this allows doctors a new way to deal with tuberculosis in situations such as multi-drug resistant TB (MDR-TB) or extensively drug resistant TB (XDR-TB). Use of directed TNF as a possible aide to the immune system for forming granulomas could pose a new treatment for patients suffering from MDR-TB or XDR-TB. The researchers were then able to understand that the loss of the TCR on macrophages was associated with granuloma disorganization. By treating granulomas with anti-TNF antibodies (which remove TNF), the researchers found that granulomas dispersed, and TCR expression was suppressed.
This study sheds new light on an old foe. The classic form of treating TB with antibiotics is quickly becoming ineffective, as more programs such as directly observed treatment short-course (DOTS) are being used by the WHO and PAHO the costs of treatment and the pharmaceuticals are skyrocketing. This study, and the information that it sheds on macrophages and their role in granuloma formation (along with how the TCR and TNF affect granuloma formation) can have a huge impact in future research. It is a possibility that treatment of a patient with targeted TNF could cause an increase in TCR expressing macrophages prompting more granuloma formation and less granuloma dissolution. While the sample size of this study was small, usually around 10 patients, it signals a necessity for more research in this field. TB is on its way to become a global pandemic: it is highly infectious and fatal. Research must continue on non-classical (drug) treatments of TB and the immune response.
Beham AW , Puellmann K , Laird R , Fuchs T , Streich R , et al. 2011 A TNF-Regulated Recombinatorial Macrophage Immune Receptor Implicated in Granuloma Formation in Tuberculosis. PLoS Pathog 7(11): e1002375. doi:10.1371/journal.ppat.1002375
Edward E. Telzak, M.D., Kent Sepkowitz, M.D., Peter Alpert, M.D., Sharon Mannheimer, M.D., Franz Medard, M.D., Wafaa El-Sadr, M.D., Steve Blum, Ph.D., A. Gagliardi, M.D., Nadim Salomon, M.D., and Glenn Turett, M.D. (1995) Multidrug-Resistant Tuberculosis in Patients without HIV Infection.
Glaziou P, Floyd K, Raviglione M (2009) Global burden and epidemiology of tuberculosis. Clin Chest Med 30: 621–636, vii.
Knechel, Nany (2009) Tuberculosis: Pathophysiology, Clinical Features, and Diagnosis. Critical Care Nurse: 29:34-43.