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mechanism by which NK cells become inactivated has been examined. Previous
reports on the inactivation of NK cells by target cells used unfractionated or
puri®ed NK cells and did not attribute whether the inactive phenotype repre-
sented all of the NK cells or it was restricted to a particular subset of the
NK cells. We have established the presence of three functionally distinct sub-
populations of NK cells based on their ability to recognize, bind, and kill the
target cells. These subsets consist of nonconjugate-forming free cells, conjugate-
forming binding cells, and killer cells (Lebow and Bonavida, 1990). Using these
subsets, we demonstrated that the inactivation of NK by target cells was re-
stricted to the NK conjugate subset, whereas the nontarget binding free NK
subset was not inactivated and responded to IL-2 (Jewett and Bonavida,
1995b). Further, puri®ed cell-sorted NK-target conjugates were divided into
two fractions: in one fraction, the NK cells were dissociated from the target
cells (NK
DC
), and in the other fraction, the conjugates were not disturbed
(NK
C
). After incubation overnight with IL-2, the cytotoxic function of NK
C
was not augmented, although a subpopulation proliferated and secreted TNF-a
and IFN-g into the supernatant. In contrast, NK
DC
cytotoxic activity was
enhanced by IL-2, but proliferated poorly and did not secrete TNF-a and
IFN-g following IL-2 activation. The phenotype of the inactive NK was
CD16
dim=ÿ
CD65
2
CD16
2
. Further, a signi®cant fraction of NK cells in the
NK
C
was programmed for cell death by apoptosis. These ®ndings reveal that
sensitive target cells mediated inactivation of NK cells for cytotoxicity resulting
in loss of NK cells (Jewett and Bonavida, 1996). Analysis of the levels of DNA
fragmentation in the free and conjugate subsets of NK cells established that the
conjugate subset had the highest hypodiploid DNA population (i.e., undergoes
apoptosis).
In our studies, as a consequence of their interaction with either NK-sensitive
K562 target cells or ADCC target cells, the FCRgIII CD16 receptor is
down-modulated. Although a direct causal relationship between CD16 down-
modulation and induction of inactivation and anergy has yet to be established,
several lines of evidence point to the importance of this receptor in the induc-
tion of NK cell inactivation and its role in the programmed death of NK cells.
In fact, our studies with normal NK cells and NK from HIV-infected individ-
uals showed that sorted CD16
ÿ
CD56
G
cells were functionally anergic and
further that the CD16
ÿ
CD56
ÿ
population underwent programmed cell death
(Jewett et al., 1997).
Anergy in NK Cells from HIV-Infected Individuals. Depressed NK cyto-
toxicity and depletion of circulating NK e¨ector cells have been documented
in asymptomatic and symptomatic HIV-infected individuals ( Bonavida et
al., 1986; Katz et al., 1987; Mansour et al., 1990; Plaeger-Marshall et al., 1987;
Vuillier et al., 1988). The NK cell decline might be explained by the decrease in
the number of NK cells and/or by down-regulation of the expression of the NK
phenotypic markers CD16 and CD56 surface antigens. In fact, we have re-
ported that as disease progresses, the frequency of peripheral blood NK cells
