REVIEWHLA class I defects in malignant lesions:
What have we learned?
Chien-Chung Chang, Michael Campoli and Soldano Ferrone
Department of Immunology, Roswell Park Cancer Institute, Buffalo, NY, USA(Received for publication on June 9, 2003)

Abstract. Depending on the tumor types, HLA class I antigen downregulation or loss has been foundin 16% to 50% of malignant lesions in many malignancies with a clinical association with histopathological markers of poor prognosis of the disease and with reduced free interval and survival. Theseﬁndings may reﬂect the escape of tumor cells with HLA class I abnormalities from recognition anddestruction by HLA class I-restricted, tumor-associated antigen-speciﬁc cytotoxic T lymphocytes. Thispossibility has stimulated investigations on the mechanisms underlying HLA class I antigen abnormalities in malignant cells. Distinct molecular defects underlying an abnormal HLA class I phenotype havebeen identiﬁed and characterized. These defects range from structural alterations of the genes which
encode HLA class I antigen subunits to deregulation of antigen processing machinery components responsible for a functional HLA class I antigen expression. These ﬁndings, in conjunction with those ofclinical recurrence of lesions with HLA class I antigen loss following T cell-based immunotherapy inpatients, suggests that immunoselection may play a role in the generation of malignant lesions with
HLA class I antigen abnormalities. This possibility has stressed the need to effectively monitor functional HLA class I antigen expression in malignant lesions in the application of T cell-based immunotherapy as well as to develop strategies to circumvent the negative impact of immunoselection. (Keio J
Med 52 (4): 220–229, December 2003)
Key words: HLA class I antigens, malignancy, immunoselection, immunomonitoring

Introduction
HLA class I antigens are part of a highly polymorphic antigenic system and play a crucial role in theinteraction of malignant cells with immune cells. They
present peptides derived from tumor-associated antigens (TAA) to TAA-speciﬁc cytotoxic T lymphocytes(CTL) and modulate the interaction of natural killer(NK) cells and T cells with tumor cells (Fig. 1).1,2 The
appreciation of the major role played by HLA class I
antigens in the interaction of tumor cells with host’s
immune system has stimulated interest in the characterization of the molecular mechanisms, functional signiﬁcance and clinical relevance of HLA class I antigen
abnormalities which are frequently associated with malignant transformation of cells.3 In this paper, we willﬁrst discuss the molecular defects underlying HLA class
I antigen abnormalities in malignant cells. Second, we

will summarize the available information about the frequency of HLA class I antigen abnormalities in malignant lesions. Third, we will discuss the role of immune
selective pressure, imposed by the host’s immune system, in the generation of malignant lesions with HLAclass I defects. Lastly, we will discuss the potential clinical signiﬁcance of HLA class I antigen abnormalities inmalignant lesions and their impact on the outcome of T
cell-based immunotherapy of malignant diseases.
Molecular Abnormalities Underlying HLA Class I
Antigen Defects
Distinct molecular mechanisms have been found tounderlie the various abnormal HLA class I antigen phenotypes (Fig. 2) which have been identiﬁed by immunohistochemical (IHC) staining of surgically removed

Presented at the 1254th Meeting of the Keio Medical Society in Tokyo, May 22, 2002.
Reprint requests to: Dr. Soldano Ferrone, Department of Immunology, Roswell Park Cancer Institute, Elm and Carlton Streets, Buffalo, NewYork 14263, USA, telephone: (716) 845-8534, facsimile: (716) 845-7613, e-mail: soldano.ferrone@roswellpark.org
220

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Fig. 1 Generation and interaction of HLA class I antigen-peptide complexes with T cells and NK cells. Intracellular protein antigens, which
are mostly endogenous, are marked for ubiquitination within the cytosol and subsequently degraded into peptides by proteasomal cleavage.
The constitutive proteasome subunits delta, MB1 and Z and the interferon-g inducible immunosubunits LMP2, LMP7 and LMP10 are responsible for the catalytic activity of the proteasome. Once generated, peptides are transported into the endoplasmic reticulum through the dimerictransporter associated with antigen processing, TAP1 and TAP2. TAP is responsible for both qualitative and quantitative peptide translocation.
Nascent HLA class I antigen heavy chains are synthesized in the ER and associate with the chaperone immunoglobulin heavy chain binding
protein (BiP), a universal ER chaperone involved in the translation and insertion of proteins into the ER. Following insertion into the ER, theHLA class I heavy chain associates with the chaperone calnexin and the thiol-dependant reductase ERp57. Calnexin dissociation is followed by
HLA class I heavy chain association with b 2 m, tapasin and the chaperone calreticulin. Calnexin, calreticulin and ERp57 play a role in folding of
the HLA class I heavy chain. Subsequently, tapasin brings the HLA class I heavy chain, b2 m, chaperone complex into association with TAP andplays a role in both quantitative and qualitative peptide selection. The trimeric HLA class I-b2 m-peptide complex is then transported to theplasma membrane where it plays a major role in the interactions of target cells with (a) peptide-speciﬁc CTL through TCR and with (b) NK cellthrough inhibitory receptor KIR.

malignant lesions with monoclonal antibodies (mAb)(for review, see ref. 3).
Total HLA class I antigen loss is caused by loss of b2 microglobulin (b2 m) expression and/or function. As aresult, the HLA class I heavy chain-b2 m-peptide complex is not formed and not transported to the cell
membrane (Fig. 2A). The mutations identiﬁed thus
far in b2 m genes range from large deletions to single
nucleotide deletions, which in most cases inhibit thetranslation of b2 m mRNA.4–7 Recently, a novel point
mutation resulting in a single amino acid substitution
in codon 25 of b2 m (C25W) has been identiﬁed in themelanoma cell line Vmm5B with total HLA class I antigen loss (manuscript in preparation). This mutation
abolishes the disulﬁde linkage required for the native
structure of b2 m and prevents its binding to HLA class I
heavy chains. Although the mutations are distributed
randomly in b2 m genes, a mutation hotspot has beensuggested to be located in the CT repeat region in exon
1 of the b2 m gene. Mutations in this region have been
identiﬁed in more than 75% of tumor cells with total
HLA class I antigen loss4 and have been found to parallel the mutator phenotype in tumor cells,8 reﬂecting
the increased genetic instability of this region during

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Chang CC, et al: HLA class I defects and cancer

Fig. 2 Molecular mechanisms underlying abnormal HLA class I antigen phenotypes identiﬁed in malignant cells. (A) Total HLA class I antigen (HLA-A, -B and -C) loss is caused by loss of b 2 m expression and/or function. An example is represented by the large deletion of the b 2 mgene detected by PCR in the melanoma cell line FO-1 as compared to the HLA class I-positive melanoma cell line Colo38. (B) Selective HLA
class I allospeciﬁcity loss is caused by loss of the gene(s) which encode the lost HLA class I allele(s) or by mutations which inhibit their transcription or translation. An example is represented by the lack of HLA-A2 antigen-speciﬁc mAb reactivity with the melanoma cell line T372A
which has selective HLA-A2 antigen loss. (C) Total loss of one HLA class I haplotype is caused by total or partial loss of one copy of chromosome 6 which encodes the genes for HLA class I heavy chains. An example is represented by the loss of HLA-A24, -B49, -Cw7 in the melanoma cell line COPA-159, as determined by comparative HLA class I genotyping of the tumor cells and the patient’s lymphocytes. (D) Total
HLA class I antigen downregulation is caused by loss or downregulation of antigen processing machinery components. An example is represented by TAP1 loss in the melanoma cell line SK-MEL-19, as determined by intracellular cytoﬂuorograghic analysis. (E) Selective HLA class I
locus downregulation may be caused by locus-speciﬁc defects in HLA class I gene transcription. An example is represented by HLA-A antigen
downregulation on the melanoma cell line Om431, as assessed by ﬂow cytometry.

malignant transformation of cells.8 It is noteworthy thatb2 m defects leading to total HLA class I antigen loss bymalignant cells result from two events: mutations in one
copy of the b2 m gene and loss of the other copy. The
latter event is often referred to as loss of heterozygosity(LOH), a notable genetic abnormality frequently foundin malignant cells (Fig. 3A).9 The requirement of two
events to generate total HLA class I antigen loss inmalignant cells accounts for the lower frequency of thisphenotype than that of most of the other phenotypes
found in malignant cells.3
Selective HLA class I allospeciﬁcity loss, e.g. HLAA2 loss, is caused by loss of the gene(s) which encode

the heavy chain of the lost HLA class I allele(s) or by
mutations which inhibit its (their) transcription or translation (Fig. 2B).4 Unlike total HLA class I antigen loss
which requires two mutational events in the b2 m gene,
selective HLA class I antigen loss results from only one
mutational event in a heterozygous allelic background(Fig. 3B). Whether a mutation hotspot in the genes
encoding HLA class I heavy chains exists remains to be
determined.
Loss of one HLA class I haplotype, e.g. HLA-A24,-B56, -Cw7, appears to be frequently caused by lossof segments of the short arm of chromosome 6 where
HLA class I genes reside (Fig. 2C).10 This phenotype

Keio J Med 2003; 52 (4): 220–229

223

Fig. 3 Contributions of loss of heterozygosity (LOH) to the generation of abnormal HLA class I phenotypes in malignant cells. (A) Total
HLA class I antigen loss as a result of b 2 m gene mutations requires two genetic events, one of which involves a mutation in one copy of thewild-type b 2 m gene and the other involves loss of the non-mutated copy, i.e. LOH. (B) Selective HLA class I allospeciﬁcity loss, e.g. HLA-A2
antigen loss, however, requires only one genetic event which involves mutations of the HLA-A2 gene. The other allele, HLA-A26, remains
intact; no LOH is required. (C) Total HLA class I downregulation as a result of TAP1 downregulation usually involves transcription repression
in both copies of the TAP1 gene, presumably by lack of transcription factor binding to the TAP1 promoter. This phenotype requires neither
structural defects nor LOH of the gene.

is often identiﬁed by HLA class I genotyping and LOH
analysis of chromosome 6. LOH at chromosome 6
appears to represent a frequent mechanism contributing to selective HLA haplotype loss in tumors.11 Thisﬁnding may reﬂect the frequent genetic recombination events at the human MHC located at chromosome
6p21.3, which carries the highest density of genes among
all gene loci in human chromosomes.12
Total HLA class I downregulation is caused by multiple mechanisms. First, transcriptional activity of HLAclass I heavy chain genes can be suppressed by thepresence of silencer located at the distal promoter11
or by epigenetic mechanisms such as hypermethylation
and/or altered chromatin structure of the HLA class
I heavy chain gene promoters.14,15 Second, the levelof HLA class I antigens expressed on cells can be
reduced by downregulation or loss of antigen processing machinery components (Fig. 2D).16 These components play a crucial role in the assembly and expressionof functional HLA class I antigen-peptide complexes(Fig. 1). Among the antigen processing machinery components, TAP1 has been most extensively investigated.
TAP1 downregulation or loss has been found in headFig. 4 Frequency of HLA class I antigen and TAP1 downregulationin malignant lesions of different embryological origin. The most common types of solid tumors for which more than 300 or 30 lesions havebeen analyzed for HLA class I antigen or TAP1 expression, respectively, are shown. (p) Indicates total HLA class I antigen downregulation; (r9) indicates selective HLA class I allospeciﬁcity loss;and (l) indicates TAP1 downregulation. Figures indicate the numberof lesions analyzed. ND: not determined. Data has been adapted fromref. 3.

and neck squamous cell carcinoma (HNSCC), in carcinomas of the breast, small cell lung (SCLC), colon,
kidney, cervix and prostate and in cutaneous melanoma
with a frequency ranging from 10–84% (Fig. 4).16,17

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TAP1 downregulation or loss is likely to be caused byabnormalities in regulatory mechanisms (Fig. 3C), since
in some instances they can be corrected by in vitro administration of cytokines, such as IFN-g and TNF-a,and is accompanied by an increase in HLA class I
antigen expression.18,19 To the best of our knowledge,
structural defects in TAP1 as a result of mutations havebeen observed only in two human tumor cell lines.20,21
The role played by other antigen processing machinerycomponents in HLA class I antigen downregulation intumor cells is under investigation and remains to beelucidated.
Selective downregulation of the gene products ofone HLA class I locus can be caused by alterations in
the transcription factors for genes encoding HLA class
I heavy chains.22,23 However, there is limited information regarding selective downregulation of the geneproducts of one HLA class I locus, since the expression of some HLA class I allospeciﬁcities in malignant
lesions has not been assessed because of the lack ofappropriate probes.
Detection and Frequency of HLA Class I Antigen
Abnormalities in Malignant Lesions
A large number of lesions removed from patientswith different types of tumors have been analyzed over
the years. The most common types of solid tumors forwhich more than 100 surgically removed primary lesions
have been analyzed include HNSCC, carcinomas of thebreast, lung, colon, kidney, cervix and prostate andcutaneous melanoma (Fig. 4).3 The frequency of total
HLA class I antigen loss and/or downregulation in primary lesions ranges from 16% for cutaneous melanoma
to 50% for prostate lesions.3 The frequency of selective HLA class I allospeciﬁcity loss or downregulation
ranges from 4% to 35% for lung carcinoma and cutaneous melanoma, respectively.3 It should be noted thatthe reported frequency of HLA class I allospeciﬁcity
loss or downregulation may be an underestimation,
since at present only a limited number of mAb to HLA
class I allospeciﬁcities suitable for use in IHC stainingof malignant lesions are available.
HLA class I antigen downregulation or loss has also
been described in other tumor types. However, thenumber of lesions that have been analyzed is too
low for one to draw deﬁnitive conclusions. These typesof tumors include stomach,24 pancreatic,25 bladder,26
germ cell,27 and basal cell28 carcinomas. Liver carcinoma29–31 and leukemia32 are exceptions to the general ﬁnding of HLA class I antigen downregulation orloss in malignant lesions. In the latter, defects in HLAclass I antigen expression in malignant cells have been
only occasionally identiﬁed. This ﬁnding is not likely to
reﬂect a lack of genetic instability in leukemic cells,

Chang CC, et al: HLA class I defects and cancer

since like solid tumor cells, leukemic cells harbor many
genetic and/or epigenetic alterations in their DNA.33
Furthermore, in view of the role of immunoselection in
the generation of malignant cell populations with HLAclass I defects,34,35 as it will be discussed later, lack ofimmune responses against leukemic cells is unlikely
to be the mechanism. This possibility is supported by
the higher frequency of HLA class I antigen abnormalities in sporadic diffuse large cell lymphoma than in
immunodeﬁcient and transplant-related lymphomas.36
Therefore, we favor the possibility that the lack ofdefects in HLA class I antigen expression identiﬁed inleukemia reﬂects the time interval between the onset ofleukemia and its diagnosis, which is likely to be shorter
than that of solid tumors. A short time interval between
the onset of leukemia and diagnosis may not allow sufﬁcient time for cells to acquire mutations in the gene(s)involved in HLA class I antigen expression and for selective pressure to facilitate the expansion of malignant
cells with HLA class I abnormalities. In liver, normal
hepatocytes, which do not express or express very low
levels of HLA class I antigens,33 acquire the expressionof these antigens during malignant transformation. The
results obtained with liver carcinoma cell lines suggestthat HLA class I antigen upregulation may result fromthe induction of antigen processing machinery components by cytokines secreted by immune cells inﬁltrating
malignant lesions.19
Role of T Cell Selective Pressure in the Generation of
Lesions with HLA Class I Antigen Defects
Generation of cells with HLA class I antigen defects
results from mutations in the gene(s) which are involvedin the expression of HLA class I antigens. It is likelythat these mutations occur randomly due to increased
epigenetic changes and genomic instability in the earlystages of tumor development.8 One important question
to ask is which mechanism(s) play(s) a role in the expansion of cells with HLA class I defects in malignant
lesions. In view of the continuous exposure of tumor
cells to the host’s immune response,34,35 one might ask
whether immune selective pressure plays a major rolein the expansion of cells with HLA class I antigen
defects so that they become the major population in alesion. As shown in Fig. 5, one can envision two possible scenarios: (i) if immune selective pressure plays amajor role, then tumor cells with HLA class I defects
expand because of escape from host’s immune response
which targets tumor cells without HLA class I antigen
defects; (ii) if on the other hand, immune selective
pressure does not play a role, then the expansion oftumor cells with HLA class I antigen defects is independent of the development of an immune response in
the host. The available evidence derived from studies

Keio J Med 2003; 52 (4): 220–229

in animal model systems and in patients treated with T
cell-based immunotherapy argues in favor of a majorrole played by immune selective pressure in the generation of malignant lesions with HLA class I antigen
defects.37–39
If immune selective pressure does play a major rolein the generation of malignant lesions with HLA class
I antigen defects, how does the host’s immune system react when tumor cells have developed an escapemechanism to the ongoing immune response? Does theFig. 5 Models of the role of T cell immunoselection in the generationof malignant lesions with HLA class I antigen abnormalities. (A) If T
cell selective pressure plays a role in the generation of malignant
lesions with HLA class I antigen loss, it is expected that tumor cellswith HLA class I antigen expression will be selectively targeted anddestroyed with the emergence and outgrowth of tumor cells without
HLA class I antigen expression. (B) If T cell selective pressure does
not play a role in the generation of malignant lesions with HLA class I
antigen loss, then the emergence of tumor cells without HLA class I
antigen expression will be independent of the presence of T cell selective pressure.

225

immune system change the target or is the immune
system unable to mount an immune response to a different target? To the best of our knowledge, no studies
have addressed these questions. The frequent detectionof multiple HLA class I antigen defects in cell lines
isolated from malignant lesions argues in favor of thepossibility that a patient’s immune response changes its
target when a tumor cell population develops an escapemechanism. As illustrated in Fig. 6, when tumor cells
develop an escape mechanism to evade an ongoing immune response targeted on immunodominant antigens,the host’s immune system redirects its immune response
toward subdominant antigens. This change in selective
pressure will favor the expansion of a new populationof tumor cells capable of escaping the new immune
response. Subsequently, tumor escape variants, whichhave been generated in the early stages of immune response, would now be targeted by immune response at
a later stage. Following these sequential phases of immune response, tumor cells with multiple defects would
eventually be generated. If this interpretation is correct,
a tumor would grow only when it has developed enoughescape mechanisms to avoid the range of immune responses a patient has been able to mount against his
own tumor.
Clinical Signiﬁcance of HLA Class I Abnormalities
in Malignant Lesions
Abnormalities in HLA class I antigen expression inmalignant lesions appear to have clinical signiﬁcance,since they are associated with histopathological characteristics of the lesions and/or with clinical parameters
in several malignant diseases.3 As already mentioned,in general the frequency of HLA class I antigen defects in metastatic lesions is higher than that in primaryFig. 6 Sequential immunoselection as a proposed mechanism for the generation of malignant cells with multiple HLA class I antigen defects.
In the early stages of T cell-mediated immune response, tumor cells with the immunodominant HLA-A2-TAA peptide complex expression are
targeted and destroyed, which results in the emergence and expansion of tumor cells without HLA-A2-TAA peptide complex expression. In
response, the immune system redirects its immune response toward the subdominant HLA-A3-TAA peptide complex expressed by this tumor
cell population, which results in the selection and expansion of tumor cells with neither HLA-A2-TAA peptide complex nor HLA-A3-TAA
peptide complex expression. Eventually, tumor cells with multiple HLA class I antigen defects are generated.

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Chang CC, et al: HLA class I defects and cancer

Table 1

Association of HLA Class I Antigen Defects with the Clinical Course of Malignant Diseases

Poor prognosis

Favorable prognosis

No association

HNSCC
Breast carcinoma
SCLC
Prostate carcinoma
Bladder carcinoma
Squamous cell carcinoma of the cervixa
Cutaneous melanoma

Uveal melanoma
Colon carcinomaa

NSCLC
Pulmonary adenocarcinoma
Adenocarcinoma of the cervix
Cutaneous squamous cell carcinoma
Large cell lymphomabLarge cell immunoblastic lymphomasb

a Conﬂicting reports in the literature. b B- or T-lymphomas.

and premalignant lesions.3 However, depending on thetumor type, HLA class I antigen defects can be associated, inversely correlated or not associated with disease progression and/or poor clinical outcomes (Table
1).3 The reasons for these discrepancies are not known
but may reﬂect differences in the characteristics of thepatient population, the methods of analysis and/or thesystem used to score HLA class I antigen expression. In
addition, these ﬁndings may be attributed to differencesin types of immune response elicited by tumors of different tissue or differences in routes of metastasis. An
example is represented by the opposite association of
HLA class I antigen downregulation with the clinicaloutcome in cutaneous and uveal melanomas.40 HLAclass I antigen downregulation is associated with apoor prognosis in cutaneous melanoma, where CTL are
believed to control the metastatic tumor spread via thelymphatics.40 In contrast, HLA class I antigen downregulation is associated with a favorable clinical outcome in uveal melanoma, where NK cells, which tend tokill tumor cells with a low HLA class I antigen expression,41,42 have been suggested to limit metastasis via
the blood.
The major role played by the HLA class I-TAA
peptide complex in the recognition of tumor cells by
CTL can be further illustrated by the association foundbetween abnormalities in the expression and/or function of antigen processing machinery components andpoor clinical course of the disease in some malignancies.3 This association most likely reﬂects the importance of these components in the generation offunctional HLA class I-TAA peptide complexes. Notably, TAP1 downregulation has been reported to associate with tumor staging and reduction in patients’survival in breast carcinoma, SCLC, cervical cancer
and cutaneous melanoma.3 An increased frequency of
TAP1 downregulation in metastatic lesions when compared to primary lesions has also been reported inbreast carcinoma, cervical carcinoma and cutaneous
melanoma.3 Most recently, the role of tapasin in theclinical course of malignant diseases has been suggested by Ogino et al. who reported that tapasin down-

regulation in conjunction with HLA class I antigen
downregulation was associated with reduced survival
in patients with maxillary sinus squamous cell carcinoma.43 It remains to be determined whether this ﬁnding applies to other types of tumors. Nevertheless, allof these ﬁndings are likely to reﬂect the crucial roleof TAP1 and tapasin in the generation of HLA class
I antigen-TAA peptide complexes and suggest thatalterations in the repertoire of peptides presented by
HLA class I antigens may provide an alternate routeof immune escape for malignant cells. This possibility
highlights the need to monitor speciﬁc HLA class I
antigen-TAA derived peptide complex expression inmalignant lesions. To this end, we have begun to develop probes capable of recognizing allospeciﬁc HLAclass I antigen-TAA derived peptide complex expression on malignant cells (manuscript in preparation).
Conclusion
During the past ten years our understanding of themechanisms underlying a tumor-speciﬁc immune response and tumor escape has substantially increased.
HLA class I antigen defects remain an important tumor
escape mechanism since they affect the interactions oftumor cells with HLA class I-restricted, TAA-speciﬁc T
cells in the course of a malignant disease. Although thishas not been formally proven yet, increasing ﬁndings of
HLA class I antigen loss variants in recurrent metastatic lesions in patients who had experienced clinicalresponses following T cell-based immunotherapy37,38
are compatible with a major role played by HLA class I
antigen abnormalities in tumor cell escape from TAAspeciﬁc CTL lysis. The region of chromosome 6, which
carries the human MHC,10 is unstable during malignant transformation of cells [3]. Mutations may leadto downregulation or loss of the HLA class I antigenTAA derived peptide complex which plays a key role in
the interaction of tumor cells with CTL. The selective
pressure imposed by CTL on tumor cell populations
may result in the expansion of tumor cell subpopulations which are not recognized by host’s immune system.

Keio J Med 2003; 52 (4): 220–229

227

Fig. 7 Lack of absolute correlation between expression of a HLA class I alloantigen and that of HLA class I-TAA peptide complex on tumor
cells. Detection of HLA-A2 antigen does not always correlate with expression of the HLA-A2-TAA peptide complex, which is recognized by
HLA-A2-restricted, TAA-speciﬁc CTL. (A) Both HLA-A2 antigen and HLA-A2-TAA peptide complex are detected on a tumor cell. (B)
HLA-A2-TAA peptide complexes are not expressed in spite of the detection of HLA-A2 antigen on a tumor cell. (C) Neither HLA-A2 antigen
nor HLA-A2-TAA peptide complex are expressed on a tumor cell.

From a practical viewpoint, the possible role playedby immune selective pressure in the generation of malignant lesions with HLA class I antigen defects suggests that the use of T cell-based immunotherapy forthe treatment of malignant diseases may only be successful in a limited number of cases. Even when successful, it is likely that the selective pressure imposed
by T cell-based immunotherapy will facilitate the emergence and expansion of tumor populations with HLAclass I defects and eventually the recurrence of malignant lesions. Therefore, it will be important to combine
T cell-based immunotherapy with other types of immunological and non-immunological strategies which utilize distinct mechanisms to control tumor growth. Furthermore, monitoring HLA class I antigen expression
in malignant lesions during the course of T cell-based
immunotherapy is important but may be misleading if
merely expression rather than function of HLA class I
antigens is assessed (Fig. 7). The latter can be accomplished by utilizing probes which recognize membranebound HLA class I-TAA derived peptide complexes.
The development of these probes is underway in a
number of laboratories including ours.44–46 If provenuseful in patients, these probes may greatly increase the
efﬁcacy of T cell-based immunotherapy for the treatment of malignant diseases.
Acknowledgements: This work was supported by PHS grants RO1
CA67108, P30 CA16056 (S.F.) and T32 CA85183 (M.C.) awarded by
the National Cancer Institute, DHHS and a fellowship awarded by theSusan G. Komen Breast Cancer Foundation (C.-C.C.).

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