Innate immunity is the body’s nonspecific defense against foreign agents and is a first line of defense for the respiratory tract. For example, macrophages and neutrophils are the primary cells in the lungs that provide innate immunity. Dendritic cells are also crucial in this defense mechanism due to their ability to take up and process bacteria so that specific immune responses can take place.
A type of cellular protein, called type I transmembrane receptor (TLR) is needed to recognize certain factors located on disease-causing agents (pathogens) so that the chain of events in the immune response can commence. One of these TLRs, TLR9 is a likely candidate that can respond to pathogens. Studies were conducted to determine the role of TLR9 in initiating innate immune responses against Gram-negative bacteria that cause pneumonia.
Studies were conducted in mice infected with pneumonia from exposure to the bacteria Klebsiella pneumonia. They found that dendritic cells expressing TLR9 accumulated in the lungs of these mice. However, genetically modified mice without the TLR9 expression ability had higher death rates than control mice when they were exposed to the Klebsiella pneumonia bacteria. There was decreased accumulation of dendritic cells in the TLR9 deficient mice, and immune cell (such as lung macrophages and T cells) activation was impaired. In addition, the TLR9 deficient mice could not generate the chain reaction of immune response events when exposed to the bacteria.
The bacteria continued to grow up to 50 times more in the lungs of mice that were deficient in TLR9, when compared to control mice with normal TLR9 expression. When TLR9 deficient mice were given dendritic cells taken from normal donor mouse bone marrow, the lost immunity was restored. These findings suggest that TLR9 is required for proper and effective innate immunity responses against Gram-negative bacteria infection in the lungs. Protective innate immunity in the lungs appears then to depend on the actions of TLR9 to trigger the necessary chain of events in the lung’s immune response against invading bacteria. Therefore, therapeutic approaches that enhance TLR9-based dendritic cell response may boost immunity against pneumonia.
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Bhan, Urvashi et al. "TLR9 Is Required For Protective Innate Immunity In Gram-Negative Bacterial Pneumonia: Role Of Dendritic Cells". The Journal of Immunology 179.6 (2007): 3937-3946. Web. 2 Sept. 2016.