By
From the Department of Molecular Biology, Princeton University, Princeton, New Jersey 08544
In addition to their essential role in presenting pathogen
peptides for recognition by the antigen receptor of killer
CD8 T cells, MHC class I molecules can be the targets of
another fundamentally different mode of recognition, also
involved in fighting pathogens. Studies on NK, a component of the innate immune system that comes into action at
the early phase of many intracellular infections, have uncovered an entire set of new receptors for MHC class I that
are encoded in the so-called NK gene complex (NKC; 1),
and have provided support for the idea that NK cells survey
for the absence of self-MHC molecules (2) through these
receptors (3). This function is crucial for fighting against
certain viruses, such as herpes viruses (4). The current
challenge that is addressed in this issue of The Journal of Experimental Medicine by Johansson et al., is to understand the
rules for selection and tolerance of NK cells expressing appropriate MHC-specific receptors in the face of the polymorphism of inherited MHC genes.
MHC-specific NKRs are encoded by genes in the
NK complex. They belong to at least two different structural families, one with Ig domains and the other with a carbohydrate recognition domain, that are both represented,
albeit to variable degrees, in mice and humans (7). Unlike TCRs, NKRs do not discriminate between most MHC-
peptide complexes, despite the fact that recognition is centered around the peptide-binding groove and depends upon peptide-induced conformation of the MHC molecule. In
that regard, NKR recognition of MHC is very similar to
that of antibodies against the A biochemical characteristic of many MHC-specific
NKRs is the presence of an immunoreceptor tyrosine-based inhibitory motif in the cytoplasmic tail, which binds
and activates the tyrosine phosphatase SHP-1 upon phosphorylation. Thus, normal, uninfected cells engage MHC-specific NKRs and deactivate NK cells by preventing phosphorylation-dependent cell activation transduced through
activating NK receptors such as NK1.1 (10, 11). In contrast, downmodulation of MHC class I, a strategy of escape
from CD8 killer cells attempted by many viruses, is countered by NK cell-mediated killing.
Unlike TCR genes,
NKR genes do not undergo somatic recombination, and
the number of MHC-specific NKR genes is limited, in the
range of 15-30 in mice and humans, with a degree of polymorphism that is low compared to that of MHC. Therefore,
evolutionary considerations suggest that the primary pressure for diversification has driven the MHC genes, whereas
NKR genes, which are encoded in a separate locus, had to
coevolve in an evolutionary pursuit to match the arising
new MHC alleles.
To fulfill their function of MHC surveillance at the level
of the individual, NK cells must therefore tailor the use of
their MHC-specific NKRs to the inherited set of MHC
genes through some process of selection or adaptation. Because the MHC and the NKC are encoded on different
chromosomes, and because the selection pressures for particular MHC and NKR genes may be different, exerted by
different pathogens at different times or in different populations, it may not always be possible to select a perfect NKR
repertoire. For example, some MHC alleles may not be
recognized by existing NKRs, or on the contrary, they may
be recognized with too high an affinity by some NKRs.
Some individuals may therefore inherit largely incompatible sets of alleles, with NKRs that are unable to see MHC
or that are insensitive to its downmodulation by viruses. Such theoretical situations could account for the existence
of human patients (4) or mouse strains, such as SJL (12) or nonobese diabetic (13), that are profoundly deficient in NK function.
These
evolutionary considerations emphasize the complexity of
matching the expression of NKR genes with the MHC
genes inherited by the individual. Within the framework of
the missing-self hypothesis (14), a functional NK must express one or more NKRs that bind host MHC alleles with
sufficient combined avidity to prevent autoreactivity (self
tolerance). This avidity should not be too high however, or
the NK would be unable to detect significant decreases in
MHC expression.
What controls the expression of NKR genes? A key observation is that subsets of NK can be defined by the expression of different MHC-specific NKRs (15). The frequency of NK expressing a particular MHC-specific NKR
does not vary much according to the MHC haplotype. For
example, B6 (H2b) NK cells express the Dd-specific NKR
Ly49A, a member of the Ly49 family of NKR genes, with
the same frequency (15-20%) as NK cells in B10.D2 (H2d)
mice. Thus, NK cells seem to express useful, as well as presumably useless, MHC-specific NKRs. A second important
observation is that the frequency of NK cells expressing
two different NKRs of the Ly49 family is close to what
would be expected in a purely stochastic model of Ly49
gene activation, i.e., the product of the frequency of single
expressors, suggesting that different genes and alleles are
randomly activated at a low frequency. These results
strongly support the idea that stochastic activation of Ly49
genes contributes the primary NKR repertoire (16, 17).
Since little bias is observed in the frequency of Ly49 isotype expression by NK cells of different MHC genotypes,
the possibility exists that NK cells expressing inappropriate
MHC-specific NKRs may not be deleted. It could be argued that because not all NKR specificities are known yet,
it is not possible to rule out deletion-based selection as the
main process for forging an appropriate NKR repertoire. Indeed, a definite, though modest, increase in the frequency
of expression of additional MHC-specific NKRs has been
observed in NK cells expressing a useless receptor, suggesting that at least some of the NK cells that fail to express an
appropriate NKR might be deleted.
Other observations, however, suggest that cellular adaptation rather than deletion may be
prominent in shaping the NKR repertoire. For example, it
has been observed that the surface concentration of Ly49A,
an NKR for Dd, is decreased twofold in Dd-expressing
mice (18). Conversely, many Ly49 isotypes have been shown
to be upregulated in Another form of cellular adaptation is clonal inactivation.
In this issue of The Journal of Experimental Medicine, Johansson et al. examined NK cell tolerance in transgenic mice
with a mosaic expression of the Dd. Mosaic expression of a
transgene is thought to occur as a result of stochastic gene
inactivation linked to particular sites of integration, and results in a proportion of cells that do not express the transgene. In this case, the cells do not express an MHC allele
expressed by other cells in the mouse. A somewhat similar
situation may be achieved in hemopoietic radiation chimeras reconstituted with a mixture of This striking result argues that NK cells capable of reactivity to the Dd-negative targets had not been deleted in the
mosaic mouse. Several possibilities may account for their
extremely rapid functional recovery. One possibility is that
the autoreactive NK cells were only tolerant in appearance;
when the mosaic populations were tested against MHC-negative targets, their killing ability was hidden because of
the competition by the MHC-negative cells in the mosaic.
Thus, the killing activity was revealed after removal of these
MHC-negative cells. This seems rather unlikely, since the
mosaic mice appeared to have stable proportions of MHC-positive and -negative cells. Moreover, in the case of mixed
fetal liver chimeras, the proportion of Although more refined genetic approaches are on their
way to further probe the rules of selection and adaptation
in NK cells, the results reported by Johansson et al. already
suggest that NK cells may have solved the problem of tolerance in a unique fashion. Future studies in this field may also
illuminate the intriguing observation that MHC-specific
NKRs are also expressed by other cell lineages, including
some T cell subsets (21, 25, 26) and mast cells (27).
1/
2 domains of MHC.
They recognize allelic forms of MHC molecules, often
cross-reacting onto several other MHC allotypes or isotypes.
2m-deficient mice, possibly to make up for the decreased concentration of MHC molecules
(19). It is not clear yet whether these modulations merely
result from receptor engagement, or whether they might reflect a process of receptor calibration (14). In such a calibration model, NK cells would each increase or decrease their
level of receptors so as to be unresponsive to cells expressing the baseline concentration of existing MHC class I alleles while being maximally sensitive to MHC downmodulation. It is noteworthy in the context of this hypothesis that
several potentially autoreactive T cell subsets, some of which
also express NK receptors, have been reported to express
decreased levels of TCR or CD8 coreceptor (20).
2m-sufficient and -deficient fetal liver cells (24). In both cases, NK cells appeared
to be tolerant of the cells that do not express MHC class I. However, when Johansson et al. isolated Dd-positive NK
cells from the Dd-negative NK cells and cultured them for
24 h, the cells completely recovered the ability to kill Dd-negative targets.
2m-positive and -negative hemopoietic cells roughly corresponded to that used
for reconstitution, arguing against a continuous attack of
the latter by NK cells. Another possibility is that the NK
cells were `anergic' or desensitized to the absence of Dd,
and could very promptly recover in the absence of the
tolerogen. Finally, existing functional NK cells may have
adapted to their new environment by recalibrating their
various NK receptors to match the new MHC levels. As
discussed above, the latter possibility might be difficult to
test at present because subtle adjustments of the expression
of several MHC-specific receptors might be involved.
Address correspondence to Dr. Albert Bendelac, Department of Molecular Biology, Princeton University, Washington Road, Princeton, NJ 08544. Phone: 609-258-5454; FAX: 609-258-2205; E-mail: abendelac @molbiol.princeton.edu
Received for publication 19 June 1997.
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