NK Cells and T Cells Share Some Features

NK cells are lymphoid cells derived from bone marrow that share a common early progenitor with T cells, but their detailed lineage remains to be worked out. They express some membrane markers that are found on monocytes and granulocytes, as well as some that are typical of T cells. Different NK cells express different sets of membrane molecules. It is not known whether this heterogeneity reflects subpopulations of NK cells or different stages in their activation or maturation. Among the membrane molecules expressed by NK cells are CD2, the 75-kDa p subunit of the IL-2 receptor, and, on almost all NK cells, CD16 (or Fc^RIII), a receptor for the Fc region of IgG. Cell depletion with monoclonal anti-CD 16 antibody removes almost all NK-cell activity from peripheral blood.

Virus-specific CTLs

Virus-specific CTLs

Viral Infection Steps

3 4 5 6 7 8 9 10 11 12 Days after viral infection

FIGURE 14-12

3 4 5 6 7 8 9 10 11 12 Days after viral infection

FIGURE 14-12

Time course of viral infection. IFN-a and IFN-p (dashed curve) are released from virus-infected cells soon after infection. These cytokines stimulate the NK cells, quickly leading to a rise in the NK-cell population (blue curve) from the basal level. NK cells help contain the infection during the period required for generation of CTLs (black curve). Once the CTL population reaches a peak, the virus titer (blue area) rapidly decreases.

Despite some similarities of NK cells to T lymphocytes, they do not develop exclusively in the thymus. Nude mice, which lack a thymus and have few or no T cells, have functional NK-cell populations. Unlike T cells and B cells, NK cells do not undergo rearrangement of receptor genes. This is demonstrated by the observation that NK cells develop in mice in which the recombinase genes RAG-1 or RAG-2 have been knocked out. Furthermore, while no T or B cells are found in SCID mice, functional populations of NK cells can be readily demonstrated. The power of NK cells and other protective mechanisms of innate immunity to protect animals totally lacking in adaptive immunity is nicely illustrated by the family of RAG-1 knockout mice shown in Figure 14-13.

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