Antigen Presenting Cells Have Characteristic CoStimulatory Properties

Only professional antigen-presenting cells (dendritic cells, macrophages, and B cells) are able to present antigen together with class II MHC molecules and deliver the co-stimulatory signal necessary for complete T-cell activation that leads to proliferation and differentiation. The principal co-stimulatory molecules expressed on antigen-presenting cells are the glycoproteins B7-1 and B7-2 (see Figure 10-13). The professional antigen-presenting cells differ in their ability to display antigen and also differ in their ability to deliver the co-stimulatory signal (Figure 10-18).

Dendritic cell

Dendritic cell

Class I MHC

Class II MHC

Class I MHC

Class II MHC

Macrophage Resting Activated

Macrophage Resting Activated

Class I MHC

Class II MHC

Class I MHC

Class II MHC

Class I MHC

Resting

B Lymphocyte

Activated

Class I MHC

Resting

B Lymphocyte

Activated

Class I MHC

Langerhans Cells

Class II MHC

Class II MHC

Class II MHC

Class II MHC

Antigen uptake

Endocytosis phagocytosis

(by Langerhans cells)

Phagocytosis

Phagocytosis

Receptor-mediated endocytosis

Receptor-mediated endocytosis

Class II MHC expression

Constitutive (++)

Co-stimulatory activity

T-cell activation

Naive T cells Effector T cells Memory T cells

Effector T cells Memory T cells

Effector T cells Memory T cells

Naive T cells Effector T cells Memory T cells

FIGURE 10-18

Differences in the properties of professional induce T-cell activation. Note that activation of effector and memory antigen-presenting cells affect their ability to present antigen and T cells does not require the co-stimulatory B7 molecule.

Dendritic cells constitutively express high levels of class I and class II MHC molecules as well as high levels of B7-1 and B7-2. For this reason, dendritic cells are very potent activators of naive, memory, and effector T cells. In contrast, all other professional APCs require activation for expression of co-stimulatory B7 molecules on their membranes; consequently, resting macrophages are not able to activate naive T cells and are poor activators of memory and effector T cells. Macrophages can be activated by phagocytosis of bacteria or by bacterial products such as LPS or by IFN-7, aTH1-derived cytokine. Activated macrophages up-regulate their expression of class II MHC molecules and co-stimulatory B7 molecules. Thus, activated macrophages are common activators of memory and effector T cells, but their effectiveness in activating naive T cells is considered minimal.

B cells also serve as antigen-presenting cells in T-cell activation. Resting B cells express class II MHC molecules but fail to express co-stimulatory B7 molecules. Consequently, resting B cells cannot activate naive T cells, although they can activate the effector and memory T-cell populations. Upon activation, B cells up-regulate their expression of class II MHC molecules and begin expressing B7. These activated B cells can now activate naive T cells as well as the memory and effector populations.

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