VMC 221: Veterinary Immunology and Serology (1+1)

THE TUBERCULIN REACTION

• Tuberculin is used to test/identify animals suffering from tuberculosis.
• Tuberculin is the extracts of Mycobacterium tuberculosis, M. bovis or M. avium.
• Several types of tuberculin are used. The most predominant is purified protein derivative (PPD) tuberculin prepared by growing organisms in synthetic medium, killing them with steam and filtering.
• The PPD tuberculin is precipitated from this filtrate with trichloracetic acid, washed and finally resuspended in buffer for use. Its antigenic component is thought to be heat shock protein 65 (HSP 65).
• When tuberculin is injected intradermally into a sensitized animal, a red indurated (hard) swelling slowly develops at the injection site. The inflammation begins between 12 and 24 hours, reaches its greatest intensity by 24 to 72 hours and may persist for several weeks before gradually fading.
• T cells mediate the tuberculin reaction. When an animal is invaded by M. tuberculosis, the organisms are readily phagocytosed by macrophages. Some of this antigen is presented to Th1 cells, triggers an immune response and generates memory cells. These long lived memory T cells are able to respond to mycobacterial antigen entering the body by any route.
• In a sensitized animal, on intradermal injection, the Langerhans cells take up antigen to draining lymph node and present to memory T cells and that attract TH1 effector cells.
• Circulating TH1 cells recognize the antigen, become activated and accumulate around the antigen deposit, secrete biologically active substances like IFN- g and IL-2, serotonin, IL-8 and lymphotactin (a chemotactic for lymphocytes) etc. This causes infiltration and attracts more T cells (CD 4+ and CD 8+).
• Macrophages accumulated in the lesion ingest and eventually destroy the antigen.