Immune Mechanisms against Intracellular Pathogens


Chronic infectious diseases in humans are caused by microorganisms that reside within phagocytes and epithelial cells. Host defence mechanisms against intracellular pathogens are primarily orchestrated by T lymphocytes via cytokine networks. Dysregulation of host responses lead to persistence of infection and immunopathology.

Keywords: immune response; intracellular pathogens; human pathogens; regulatory T cells; cytokines in infectious diseases

Figure 1.

DC‐SIGN of HIV favours infection of CD4 T cells: DC‐SIGN a C‐type lectin present on dendritic cell recongizes viral envelope glycoprotiens – gp 120. Immature DCs capture HIV at entry sites, achieve maturation and transmit the virus to CD4 T cells in the periphery. Low levels of HIV evades intracellular processes and degradation by entry through DC‐SIGN.

Figure 2.

Recognition/entry of Mycobacterium tuberculosis in dendritic cells is through diverse receptors: (a) It is targeted to the lysome by entry through mannose receptors, undergoes degradation and releases ManLAM. (b) Recogition by TLRs (Toll‐like receptors) results in activation of NF‐κB leading to immune activation. (c) DC‐SIGN binding high levels ManLAM results in immune suppression and deviation of intracellular killing. DC‐SIGN thus favours mycobacterial survivals whereas TLR aids in the host immune response.

Figure 3.

Cellular uptake of larger particles and pathogens (red bar) and of fluid or smaller particles (green dots). In phagocytosis the cell membrane is double and particles are trapped by circumferential pseudopods (convential). In coiling phagocytosis unilateral pseudopodia replicate and form tight whorls around the pathogen(s). In ‘loop’ phagocytosis irregular loop‐like pseudopods are put out by the host cell to capture the organism in a spacious phagosome. Whereas the phagosome tightly surrounds the organism in the former, in the ‘loop’ form it is spacious and looses around the organism. In macropinocytosis a single membrane fold bends towards the cell surface, trapping pericellular fluid.

Figure 4.

Role of accessory cells (monocytes (MO)/dendritic cells (DC)) in the differentiation of T‐helper cells: During the immune response/infection, accessory cells release diverse cytokines which lead to differentiation of specific subsets of Th cells, each with distinct function. Th1, Th2 are effector T cells whereas Th17 and Foxp3 have immune regulatory role of enhancing or suppressing immune responses respectively.

Figure 5.

Foxp3 T cells in the skin lesion of lepromatous leprosy patient detected by immunohistochemistry with specific mouse antihuman Foxp3 antibody and subsequently stained by horse‐radish peroxidase polymer (HRP Polymer) with Diamino benzamidine (DAB) as a substrate (original magnification X 200).



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Further Reading

William E Paul (2003) Fundamental Immunology, 5th edn. Philadelphia: Lippin Cott‐Raven Publishers.

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Nath, Indira(Dec 2008) Immune Mechanisms against Intracellular Pathogens. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000480.pub2]