Characterisation of breast cancer infiltrates using monoclonal antibodies to human leucocyte antigens

Role of immune regulatory cells in breast cancer: Foe or friend?

Breast cancer (BC) is the most common cancer among women between the ages of 20 and 50, affecting more than 2.1 million people and causing the annual death of more than 627,000 women worldwide. Based on the available knowledge, the immune system and its components are involved in the pathogenesis of several malignancies, including BC. Cancer immunobiology suggests that immune cells can play a dual role and induce anti-tumor or immunosuppressive responses, depending on the tumor microenvironment (TME) signals. The most important effector immune cells with anti-tumor properties are natural killer (NK) cells, B, and T lymphocytes. On the other hand, immune and non-immune cells with regulatory/inhibitory phenotype, including regulatory T cells (Tregs), regulatory B cells (Bregs), tolerogenic dendritic cells (tDCs), tumor-associated macrophages (TAMs), tumor-associated neutrophils (TANs), myeloid-derived suppressor cells (MDSCs), mesenchymal stem cells (MSCs), and regulatory natural killer cells (NKregs), can promote the growth and development of tumor cells by inhibiting anti-tumor responses, inducing angiogenesis and metastasis, as well as the expression of inhibitory molecules and suppressor mediators of the immune system. However, due to the complexity of the interaction and the modification in the immune cells' phenotype and the networking of the immune responses, the exact mechanism of action of the immunosuppressive and regulatory cells is not yet fully understood. This review article reviews the immune responses involved in BC as well as the role of regulatory and inhibitory cells in the pathogenesis of the disease. Finally, therapeutic approaches based on inhibition of immunosuppressive responses derived from regulatory cells are discussed.

Adaptive immunity programmes in breast cancer

The role of the immune system in shaping cancer development and patient prognosis has recently become an area of intense focus in industry and academia. Harnessing the adaptive arm of the immune system for tumour eradication has shown great promise in a variety of tumour types. Differences between tissues, however, necessitate a greater understanding of the adaptive immunity programmes that are active within each tumour type. In breast cancer, adaptive immune programmes play diverse roles depending on the cellular infiltration found in each tumour. Cytotoxic T lymphocytes and T helper type 1 cells can induce tumour eradication, whereas regulatory T cells and T helper type 2 cells are known to be involved in tumour-promoting immunosuppressive responses. Complicating these matters, heterogeneous expression of hormone receptors and growth factors in different tumours leads to disparate, patient-specific adaptive immune responses. Despite this non-conformity in adaptive immune behaviours, encouraging basic and clinical results have been observed that suggest a role for immunotherapeutic approaches in breast cancer. Here, we review the literature pertaining to the adaptive immune response in breast cancer, summarize the primary findings relating to the breast tumour's biology, and discuss potential clinical immunotherapies.

The immunophenotype and activation status of the lymphocytic infiltrate in human breast cancers, the role of the major histocompatibility complex in cell-mediated immune mechanisms, and their association with prognostic indicators

BACKGROUND

The aims of this study were to characterize, phenotypically, the immune infiltrate in human breast cancers, to assess the activation status of tumor-infiltrating lymphocytes (TIL), and to define the association of these findings with established prognostic indicators.

METHODS

Immunohistochemistry was performed on frozen sections of 60 primary breast cancers by use of monoclonal antibodies to T lymphocytes (CD3), T-helper cells (CD4), cytotoxic T-cells (CD8), natural killer cells (CD56), interleukin-2 receptors (IL-2R), and major histocompatibility (MHC) class I antigen (HLA-ABC) and MHC class II antigen (HLA-DR).

RESULTS

All tumors stained positive for CD3, CD4 and CD8, but with marked variation in the intensity of the infiltrate. In tumors with a moderate infiltrate of TIL, there was a trend toward a greater representation of T-helper cells. However, as the intensity of TIL increased, there was a decline in the proportion of T-helper cells and a concomitant rise in the relative proportion of cytotoxic T cells. There was a relative paucity of natural killer cells. A significant association was found between the intensity of TIL and the number of positive nodes (P=.02) and the intensity of the infiltrate of both T-helper cells and cytotoxic T cells with ER expression (P=.03 and.05, respectively). Most tumors stained positive for IL-2R. The expression of IL-2R was associated with the intensity of TIL (P<.0001), T-helper cells (P<.002), cytotoxic T cells (P=.01) and natural killer cells (P=0.04), and also with the degree of lymph node positivity (P=.02) and histologic tumor grade (P=.05). MHC class II expression was variable, and a large proportion of the tumors showed limited expression in individual cancer cells. There was an association between the expression of HLA-DR in tumor cells and the activation status of TIL (P=.03).

CONCLUSION

An immune infiltrate is an invariable finding in breast cancers, and the intensity of the infiltrate is greater in node positive tumors. Additionally, TIL may well be activated, albeit partially, in most tumors, suggesting that cell-mediated immune mechanisms are functionally intact.

Therapeutic activity of NK cells against tumors

While it is generally accepted that natural killer (NK) cells, by killing tumor cells in the circulation, represent a first line of defense against metastases, their therapeutic activity against established tumors has been limited. In this review, we describe studies to improve the therapeutic effectiveness of activated NK cells in both animal models and clinical trials to better understand the biological problems that limit their effectiveness.

Evaluation of HLA-DR expression and T-lymphocyte infiltration in osteosarcoma

Although in recent years the outcome of patients with osteosarcoma has considerably been improved by combining neoadjuvant chemotherapy with radical surgery, there still remains the problem of nonresponse to chemotherapy. T-lymphocytes play a critical role in tumor immunology, and MHC molecules are of central importance in the regulation of the immune response. It is the aim of this study to investigate whether T-lymphocyte infiltration of osteosarcomas and HLA-DR expression on tumor cells and infiltrating immune cells are of predictive or diagnostic value. Expression of CD3, CD8 and HLA-II was evaluated immunohistochemically on paraffin-embedded sections of 35 patients with high-grade osteosarcoma at the time of biopsy before chemotherapy and correlated with histologic response to chemotherapy, tumor size, age, alkaline-phosphatase serum levels and duration of symptoms. Thirty-four patients with osteoblastoma (n = 7), osteoid osteoma (n = 7) or fibrous dysplasia (n = 20) served as controls. Osteosarcomas were infiltrated by CD3+ (33/35, 95%) and CD8+ T-lymphocytes (24/35, 68%), non malignant bone tumors by CD3+ in 91% (31/34) and CD8+ T-lymphocytes in 74% (25/34), respectively. T-lymphocytes were positive for HLA-DR expression in 29% (10/35) in osteosarcomas and in 11% (4/34) in non-malignant controls. Osteosarcoma cells were positive for HLA-DR in 11/35 (31%) and non-malignant tumor cells in only 9% (3/34). Therefore, HLA-DR is overexpressed in osteosarcoma (p < 0.05). HLA-DR expression on osteosarcoma cells showed a positive correlation with HLA-DR expression on lymphocytes (p < 0.001) as well as with duration of symptoms and age (p < 0.05). Response to preoperative chemotherapy, gender, tumor size and serum alkaline-phosphatase levels did not correlate with the expression of the molecules tested. Our results show that HLA-DR is overexpressed in osteosarcoma cells compared to non-malignant bone-tumors. This overexpression, however, fails to serve as a predictive marker for response to neoadjuvant chemotherapy. The same is also true for tumor-infiltrating lymphocytes expressing CD3, CD8 and HLA-DR. Increased HLA-DR expression in osteosarcoma is most likely due to the immune response against the tumor.

CD4+ T lymphocytes infiltrating human breast cancer recognise autologous tumor in an MHC-class-II restricted fashion

Tumor-infiltrating lymphocytes (TIL) were derived from primary breast tumors, metastatic lymph nodes and malignant pleural effusions from 34 patients with breast cancer. TIL were cultured for approximately 30 days and studied for phenotype, cytotoxicity, and the ability to secrete cytokines in response to autologous tumor stimulation. Tumor specimens were obtained from two different sites in 7 patients, resulting in 41 samples from which 38 TIL cultures were established. In addition to screening 38 bulk TIL cultures, TIL from 21 patients were separated into CD4+ and CD8+ subsets and extensively studied. Three CD4+ TIL were found specifically to secrete granulocyte macrophage-colony-stimulating factor and tumor necrosis factor alpha when stimulated by autologous tumor and not by a large panel of stimulators (24-34) consisting of autologous normal cells, allogeneic breast or melanoma tumors and EBV-B cells. This cytokine release was found to be MHC-class-II-restricted, as it was inhibited by the anti-HLA-DR antibody L243. These 3 patients' EBV-B cells, when pulsed with tumor lysates, were unable to act as antigen-presenting cells and induce cytokine secretion by their respective CD4+ TIL. These findings demonstrate that MHC-class-II-restricted CD4+ T cells recognising tumor-associated antigens can be detected in some breast cancer patients.

Prognostic significance of T-cell infiltrates, expression of beta 2-microglobulin and HLA-DR antigens in breast carcinoma

In this study immunohistological staining was used to assess the presence of T-cell infiltrates and the expression of beta 2-microglobulin (beta 2-m) and HLA-DR antigens on tumor cells of 75 ductal invasive carcinomas. The results were compared with the morphometric prognostic index (MPI) that seems to be the most accurate prognostic predictor. The extent of T-cell infiltrates differed widely between tumors, but statistically significant correlation was found only with the lymph node status, namely, tumors with a high degree of infiltration had predominantly negative lymph nodes and vice versa (p < 0.05). Only 19 (25.3%) out of 75 carcinomas were beta 2-m+, 34 cases (45.3%) showed heterogeneous staining pattern and 22 tumors (29.3%) were completely negative. We could not find any significant correlation between beta 2-m expression and MPI or T-cell content. While normal breast epithelium was always HLA-DR negative, tumor cells displayed positivity in 25 cases (33.3%), 5 tumors (6.7%) were completely positive and 20 tumors (26.7%) displayed only focal expression of class II antigens. This expression did not correlate with any single prognostic parameter, nor with MPI. The results suggest that T-cell infiltrates and the expression of histocompatibility antigens can not be accepted as prognostic indicators in breast carcinoma.

Flow cytometric analysis of tumour infiltrating lymphocyte activation and tumour cell MHC class I and II expression in breast cancer patients

The primary tumour cells and tumour infiltrating lymphocytes (TILs) of 31 breast cancer patients have been analysed by dual colour flow cytometry to determine whether the phenotype and/or activation status of the TILs bears any relationship to the expression of MHC antigens on the tumour cells. The phenotype and activation status of 5000 TILs were studied using Mabs to CD4, CD8, HLA DR, CD25 (the low affinity inducible IL-2 receptor) and the transferrin receptor and related to Class I and Class II MHC expression on 5000 primary tumour cells. On the tumour cells, Class I MHC expression ranged from 1-74%, averaging 12.9%. HLA DR expression ranged from 1-69% averaging 14.3%. When the phenotypic proportions of the lymphocytic infiltrate were analysed there was found to be a correlation between tumour expression of Class I MHC and the proportion of both CD4+ (P less than 0.05) and CD8+ (P less than 0.02) T cells within the tumour. No such relationship was found with the MHC Class II antigen. When TIL activation markers were analysed, the percentage of CD8+ TILs positive for HLA DR expression correlated strongly with the expression of Class I (P less than 0.001) and Class II (P less than 0.001) antigens on the tumour cells. The percentage of CD4+ TILs positive for HLA DR expression also correlated significantly, but less strongly with the expression of Class I (P less than 0.01) and Class II (P less than 0.02) antigen expression on the tumour cells. The percentage of CD4+ TILs positive for CD25 expression correlated with both Class I (P less than 0.05) and Class II (P less than 0.03) expression on the tumour cells while the percentage of CD8+ TILs positive for CD25 did not. The percentage of TILs bearing the transferrin receptor showed no measurable correlation with the expression of either class of MHC antigen on the tumour. The data suggest that MHC expression on the tumour cells has a selective effect on the response capacity of different parts of the immune system.

Protooncogene expression in subpopulations of cells from leukemia patients

This report describes a method for preserving the light scatter patterns of cells in which myc and myb expression are being measured. Exposure of cells to 1% paraformaldehyde for 72 h prior to antibody staining for myc and myb proteins preserved the light scatter patterns. Using this method, myc and myb expression was found to be highest in lymphocytes and monocytes and lowest in granulocytes. The measurement of differences in the level of expression of these genes in subpopulations of leukemia cells obtained from individual patients is possible as is assessment of the levels of expression amongst normal and leukemia cells present in the same patient.

Analysis of distribution of tumor- and preneoplasia-infiltrating lymphocytes using simultaneous Hoechst 33342 labeling and immunophenotyping

Hoechst 33342 in vivo staining was combined with immunofluorescent staining of cell surface antigens to quantify the distribution, relative to blood supply, of lymphocytes in a preneoplastic mammary lesion, the murine C4 hyperplastic alveolar nodule (HAN), and the C4 adenocarcinoma which develops from C4 HAN. The vascular supply to lymphocytes expressing Thy 1.2, L3T4, Ly2, and ASGM1 cell surface antigens was evaluated in both tissues. The distribution of ASGM1+ cells, which include natural killer cells, differed between the two tissues, being significantly increased in the 20% brightest Hoechst-stained lymphocyte fraction in HAN but not in C4 tumor. Distribution of T lymphocytes did not differ between the two tissues. The combination of in vivo Hoechst 33342 with in vitro immunofluorescence provides a simple method to evaluate the distribution with regard to blood supply of lymphocyte subsets in solid tumors and preneoplastic lesions.

Effect of IFN-gamma on the expression of MHC class I and class II antigens in a human malignant oral epithelial cell line

This study examined the expression of major histocompatibility complex (MHC) antigens in the KB human carcinoma cell line. MHC class I antigens (HLA-ABC and beta 2microglobulin) were constitutively expressed but not significantly enhanced by interferon-gamma (IFN-gamma). By contrast, MHC class II antigens were absent or expressed minimally on unstimulated KB cells, but DR and DP were significantly stimulated in a non-coordinated pattern (HLA-DR greater than -DP greater than -DQ) by IFN-gamma.

Study of the relationship between immunohistologically demonstrated lymphocytes infiltrating human breast carcinomas and patients' survival

Eighty-five breast carcinomas were immunostained for CD3-, CD4-, CD8-, CD16-, CD22-, CD38- and CD57-positive lymphocyte subpopulations. The results were related to follow-up data (median follow-up 46 months) of 74 patients regarding overall survival and 73 patients in respect to disease-free survival. Whereas the number of axillary lymph node metastases (P less than 0.01) and the hormone receptor status (P less than 0.01) resulted in significantly different survival curves for overall survival, not one of the lymphocyte subset infiltrats correlated significantly which overall survival. For disease-free survival, pT stage (P less than 0.01) and nodal (P less than 0.01) and hormone receptor status (P less than 0.05) proved to be prognostically important. However, disease-free survival was not influenced by the infiltration of any lymphocyte subset.

Flow cytometric analysis of tumour infiltrating lymphocytes in breast cancer

In 31 patients with carcinoma of the breast the phenotype and activation status of tumour infiltrating lymphocytes (TILs) was analysed by flow cytometry. The predominant cells, in all patients, were T lymphocytes and in the majority of cases CD8+ (cytotoxic/suppressor) T lymphocytes were present in greater numbers than CD4+ (helper) T lymphocytes. There was no relationship between the degree of lymphocytic infiltration and either tumour stage or grade but there appeared to be an inverse correlation with the levels of oestrogen receptor (ER) in the tumour (P less than 0.01). Both populations of T cells had significantly higher numbers of cells carrying HLA DR (class II major histocompatibility antigen) than the equivalent populations in peripheral blood from the same patient group (P less than 0.001). The transferrin receptor was found on similar numbers of CD8+ T cells in peripheral blood and among the tumour infiltrating lymphocytes while more of the CD4+ T cells infiltrating the tumour were found to carry this receptor (P = 0.034). The Tac (CD 25) antigen was also on similar numbers of CD8+ T cells from both peripheral blood and the tumour but was on fewer of the CD4+ T cells in the tumour with respect to peripheral blood (P = 0.029). In both TILs and blood lymphocytes, the Tac antigen was consistently present on greater numbers of CD4+ T lymphocytes than on the CD8+ T lymphocytes (P less than 0.001) and as this is a component of the interleukin 2 (IL-2) receptor this may be of relevance to the use of IL-2 in TIL cancer therapy.

Lacking prognostic significance of beta 2-microglobulin, MHC class I and class II antigen expression in breast carcinomas

To evaluate the impact of MHC antigen expression on the survival of patients with cancer, 77 human breast carcinomas were investigated for the expression of beta 2-microglobulin (beta 2m), HLA-A,B,C and HLA-DR. Thirty-one benign breast tumours were stained for comparison. The results for the carcinomas were related to the survival data of the cancer patients. The expression of beta 2m, HLA-A,B,C and HLA-DR was significantly lower in malignant tumours compared to the benign lesions. Whereas all benign tumours were positive for beta 2m and HLA-A,B,C and 28/31 positive for HLA-DR the following positivity rates were found in carcinomas: 74/77 for beta 2m, 57/77 for HLA-A,B,C and 10/77 for HLA-DR. The follow-up (median 45 months) of 66 cancer patients for overall survival and of 65 patients for disease-free survival revealed no influence of beta 2m, HLA-A,B,C or HLA-DR expression on the prognosis of this cancer. In conclusion, experimental data indicating the importance of MHC antigens in anti-tumour responses are not confirmed by the analysis of cancer patient survival data.

Risk factors for breast recurrence in premenopausal and postmenopausal patients with ductal cancers treated by conservation therapy

Risk factors for local failure were evaluated for 496 clinical Stage I-II patients with infiltrating ductal carcinomas (median follow-up, 71 months) treated by conservative surgery and radiotherapy. Monofactorial analysis identified the following factors to be correlated with increased risk: moderate/marked mononuclear cell reaction (MCR), high histologic grade (G), extensive intraductal component (EIC), tumor necrosis, macroscopic multiplicity, estrogen receptor negativity, anatomic tumor size, age younger than 40 years, and vascular invasion. Only MCR, G, and EIC proved significant in Cox multivariate analysis. These risk factors were highly age dependent, with EIC markedly more prevalent in women younger than 50, MCR and G in women younger than 40. Separate Cox analysis for premenopausal patients showed that MCR/EIC determined risk independent of resection margins: tumors with MCR had a 28%, and with EIC a 22% probability of recurring locally by 5 years. Premenopausal patients with neither risk factor had a very low failure rate (2.6% at 5 years), regardless of age. For postmenopausal patients risk of breast recurrence was determined both by adequacy of resection margins and grade, with a high local failure rate for patients having G3 tumors with positive or indeterminate margins (31% at 5 years). The authors conclude that the microscopic examination is the only useful tool for assessing the risk of local failure, which is quite low for the majority of patients treated with breast conservation. High-risk patients can be recognized morphologically. The age dependence of morphologic risk factors appears to explain the high local failure rate seen in patients younger than 40.

Quantitative immunohistochemical analysis of mononuclear infiltrates in breast carcinomas--correlation with tumour differentiation

Inflammatory infiltrates were analysed in tissue sections of 76 breast carcinomas by counting the percentage of macrophages, IgA+ and IgG+ plasma cells, T cells with their subpopulations, and natural killer cells, and by measuring postcapillary venules (PCVs, found in 12 cases) within the infiltrates. These parameters were correlated with nuclear grade and biochemically determined hormone receptor status, known markers of tumour differentiation. A direct correlation was found between the extent of inflammation and nuclear grade (P less than 0.0001), and an inverse correlation between inflammation and oestrogen receptor (OR) positivity (P less than 0.05) as well as inflammation and progesterone receptor (PR) positivity (P less than 0.05). The percentage of the OKT8+ suppressor/cytotoxic T cells increased when the inflammation expanded from scanty to moderate (P less than 0.02). The diameter of the PCVs also increased with increasing inflammatory infiltrate (P less than 0.02). In addition, a direct correlation exists between the diameter of the PCVs and both the percentage of the OKT8+ T cells (P less than 0.04) and the Leu-7+ natural killer cells (P less than 0.03).

Density and phenotype of tumour-associated mononuclear cells in colonic carcinomas determined by computer-assisted video image analysis

The density and phenotypes of tumour-associated mononuclear cells (TAMC) in tissue sections of colonic carcinomas was determined by the technique of video image analysis (VIA). This technique allowed an accurate and objective enumeration of both total mononuclear cells (MC) in H&E stained sections and individual types of cells as revealed by immunoperoxidase staining with monoclonal antibodies in frozen sections. This enumeration allowed reliable statistical analysis of the differences between sample groups. Using this technique it was found that the density of MC in histiologically normal tissue was significantly higher than in tumour tissue. Tumours from patients with the best prognosis (stage A) had significantly higher numbers of TAMC than stage B (P less than 0.02), C (P less than 0.002) and D (P less than 0.002) tumours. The differences in the density of TAMC between tumours obtained from stage B and C and that between C and D were not significant, whereas stage B had a significantly higher TAMC density than stage D tumours (P less than 0.05). Comparing tumour differentiation, well differentiated adenocarcinomas had a significantly higher (P less than 0.05) TAMC density than poorly differentiated tumours but not moderately differentiated tumours. Moderately and poorly differentiated adenocarcinomas did not differ significantly in the density of TAMC. In examining the phenotype of these cells, it was found that T lymphocytes formed the majority of the TAMC with the CD4+ subset predominating in 28 of 29 cases. Similarly, all sections of normal colon (taken at least 4 cm away from the tumour) had more CD4+ than CD8+ cells. The proportion of the total leucocyte population that was CD3+ was comparable in normal and tumour tissue. Generally, few macrophages were present in either tumour or normal tissues. B cells (CD21%) and subset of NK cells (CD57+) were not detected in the tumours. There were no significant differences in the proportion of leucocytes which were CD4+, CD8+ and CD14+ (macrophages) between the normal colon and the tumour tissues. The types of cells in the TAMC population did not differ with tumour stage or differentiation or with the density of the TAMC itself.

Sparse distribution of gamma/delta T lymphocytes around human epithelial tumors predominantly infiltrated by primed/memory T cells

Evidence from the mouse system has suggested that T lymphocytes accumulating in non-lymphoid tissue, in particular epithelia, may preferentially express the T cell receptor (TCR) gamma delta. In this study, we characterize the T cell receptor alpha beta or gamma delta phenotype of lymphocytes infiltrating human tumors of epithelial origin using monoclonal antibodies (mAb) for immunohistology and flow cytometry on cells extracted by enzyme digestion. This report shows that the majority of CD3+ tumor-infiltrating lymphocytes are TCR alpha beta+ but a small percentage of TCR gamma delta can be clearly defined scattered throughout the tumor tissue with apparently no microanatomical selection. So far there has been little evidence for an accumulation of activated T cells in human tumor tissues as defined by mAb against molecules appearing transiently during the acute phase of activation. Now mAb are available that can identify primed or memory T cells such as mAb UCHL-1 recognizing the CD45RO antigen. Here we show that CD3+ tumor-infiltrating lymphocytes have a statistically significant accumulation of primed T cells, as compared to the autologous peripheral blood lymphocytes, suggesting their immune stimulation by tumor cells.

Correlation of HLA-D/Ii antigen expression in breast carcinoma with local lymphohistiocytic infiltration reveals considerable dysregulation in a subset of tumors

Non-neoplastic mammary glands and 203 carcinomas of the breast were examined immunohistochemically for expression of HLA-DR, -DP, and -DQ molecules and the HLA-D-associated invariant chain (Ii). According to the reaction of normal breast epithelium in the course of chronic inflammation, these antigens were found to be induced in 120 carcinomas. In 106 of these the order was Ii greater than or equal to HLA-DR greater than or equal to HLA-DP greater than or equal to HLA-DQ. The extent of induction of these sequentially expressed antigens correlated with the density of local lymphohistiocytic infiltration (Ii: p = 0.003; HLA-DR: p = 0.0005; HLA-DP: p = 0.0000005; HLA-DQ: p = 0.01). Therefore, Ii/HLA-D antigen expression of carcinoma cells in local association with inflammation was regarded as an adequate reaction. The presence of these antigens in the absence of inflammation was regarded as "inadequate hyper-expression", and was found in 14 cases involving Ii, in 8 involving HLA-DR, and in 1 case each involving -DP and -DQ molecules. In contrast, 20 cases expressed minimal/no Ii despite heavy inflammatory infiltration. This "inadequate hypo-expression" was also found in 25 cases involving HLA-DR, in 30 involving -DP and in 40 involving -DQ determinants. Combining the 2 variants showed that at least 54 cases (26.6%) were characterized as being dysregulated. "Adequate vs. inadequate Ii/HLA-D antigen expression" did not correlate with tumor grading and might therefore be an independent parameter.

What's new in the local immune response in cancer?

Malignant tumors in humans are commonly associated with an inflammatory infiltrate. The mechanisms that account for the accumulation of T-lymphocytes and macrophages--these cells comprise the major components of tumor infiltrates--in the vicinity of a growing tumor are not fully understood. Tumor specific and immunogenic antigens could not be demonstrated in most solid tumors of humans, in contrast to several experimental tumor models. Thus it is not proven in human malignancies that neoantigens expressed on malignant cells are the signal which initiates an inflammatory response that, immunohistologically, is comparable to mononuclear infiltrates present in allograft rejection. A variety of nonspecific factors including lymphokines released by tumor cells may also account for the accumulation of inflammatory cells at the tumor site. The difficulties to evaluate the functional role of the "local immune response" for tumor and host are even greater. Most tumors progress in the presence of mononuclear infiltrates. Do they progress in spite of or because of the action of the local immune response? Clinical, immunopathological, and experimental data suggest that both is right, and that at least four distinct properties of tumor-associated immune reactions exist: Regression, Selection, Modulation and Progression. These distinct properties will be discussed below, using mainly the malignant melanoma of the skin as a model for a malignant tumor in humans.

HLA class II sublocus expression in benign and malignant breast epithelium

Using immunohistochemistry and a panel of five monoclonal antibodies, the epithelial expression of HLA class II sublocus products by benign and malignant breast has been studied. The magnitude of the stromal mononuclear inflammatory cell infiltrate was assessed. There was expression of HLA class II by 75 per cent of epithelial cells in the benign tissues, with little variation in intensity and between antibodies. There was coordinate expression of DR and DQW1. Epithelial expression by carcinomas was more complex and variable. Most (61 per cent) carcinomas exhibited variable loss of epithelial expression of class II products, as detected by three antibodies recognizing epitopes on DP, DQ, and DR together. Thirteen (28 per cent) carcinomas were completely negative or had very occasional positive cells. The extent of this loss was unrelated to the magnitude of the inflammatory infiltrate and axillary lymph node status. No well-differentiated carcinomas exhibited complete loss. Furthermore, non-coordinate expression of DR and DQW1 was present in 8 out of 40 carcinomas, with the proportion of DQW1 positive epithelium always being less than that of DR. Carcinomas exhibiting non-coordinate expression were never well differentiated; there was no relationship with the extent of the inflammatory infiltrate. This is the first study to detail HLA class II expression in breast, and our results suggest that alterations in expression of these products may modify or reflect tumour behaviour.

Inflammatory cell infiltrates in human melanoma at different stages of tumor progression

Progression of human melanoma is associated with changes in antigenic phenotypes of tumor cells. To establish whether inflammatory infiltrates in progressing melanoma also change, we studied 146 cutaneous melanomas at different stages of progression. Monoclonal antibodies (MAbs) against lymphocyte and macrophage subpopulations, interleukin-2 receptor (IL-2 R), immune interferon (IFN-gamma), and the IFN-gamma-inducible, progression-associated melanoma antigens HLA-DR and gp89 were applied in situ. During the course of melanoma progression, decreased amounts of peritumoral T cells, IL-2 R-expressing lymphocytes and dermal T6+ dendritic cells were found, while increased numbers of intratumoral T cells, inflammatory (27E10+) and mature (25F9+) macrophages were associated with local progression of primary melanomas. In metastases, most infiltrate components except 25F9+ macrophages were rare. Positive correlations were observed between: (1) dermal T6+ cells and IL-2 R+ lymphocytes, and (2) presence of IFN-gamma in the infiltrate and HLA-DR and gp89 antigens on tumor cells. In all stages, HLA-DR expression on tumor cells was correlated with: (1) a shift towards T8+ lymphocytes in the infiltrates and (2) a loss of IL-2 R expression. Our data suggest mutual influences between melanoma cells and mononuclear cell infiltrates in situ.

Loss of polymorphic A and B locus HLA antigens in colon carcinoma

In the present study we have confirmed that approximately one third of human colorectal carcinomas fail to express the HLA - A,B,C monomorphic determinant reactive with the W6/32 MAb, and 44% express class II HLA antigens as shown by reactivity with NFK-1 MAb. Reduced staining with the W6/32 MAb was not always associated with the loss of beta 2m. In addition, the expression of HLA-A2 and Bw4 class I specific haplotypes on normal colon epithelium and tumour biopsy tissue was assessed. All normal colonic epithelia stained positively with MAb against A2 and Bw4 antigens, but a loss of these determinants was shown on tumour biopsies from patients tissue typed for the respective specificities. Loss of the A2 haplotype was shown in 4 of 15 tumour tissue samples, and loss of Bw4 specificities in 5 out of 7 tissue samples. The failure to detect specific loci determinants was not necessarily associated with loss of reactivity with W6/32 MAb.

Alterations of class I HLA genes in human colon cancers

Alterations of HLA class I genes were found in 3 of 12 human colon cancers. Rearrangements in HLA class I genes were observed in 2 cancers and amplification of HLA-coding genes was observed in 1 cancer. All 3 cancers were at an advanced stage. No examples of amplification or rearrangement in the HLA genes were found in 10 other tumours of diverse types. No alterations in the beta 2-microglobulin gene were observed in 22 human solid tumours included in this study. The association between alterations in HLA genes and proto-oncogenes in these tumours is discussed.

The altered expression of MHC-class II determinants on monocytes of cancer patients

The expression of MHC class II determinants Ia.7 (detected by cross reactive mouse anti-Iak antibody) and HLA-DR on monocytes (MO) of gastric and colorectal cancer patients was examined. An increased proportion of MO bearing the Ia.7 determinant was found, while the number of MO expressing DR was not elevated. In gastric cancer patients the increased expression of the Ia.7 determinant was most pronounced in advanced cancer (stage IVA and IVB). The increased expression of this determinant was related to the presence of the tumour as the number of MO expressing Ia.7 decreased 6 months following surgical resection of the tumour. Further, the increased expression of Ia.7 on MO correlated with the tumour infiltration of the serosa. The Ia.7 determinants were mainly expressed on MO which also expressed the receptor for the Fc part of immunoglobulin. Immunostaining in cellular infiltrates surrounding the tumour revealed that Ia.7+ macrophages (MO) were more numerous than in normal gastric mucosa and severe chronic gastritis and were mostly present in close proximity to tumour cells, while DR+MO were mainly localized within the stromal tissue of the tumour and their number was not increased in cancer infiltrates. These observations indicate that the Ia.7+ subpopulation of MO may be involved in the anti-tumour response of the host.

Characterization and frequency distribution of lymphoreticular infiltrates in axillary lymph node metastases of invasive ductal carcinoma of the breast

One hundred and seventy-five axillary lymph nodes containing metastatic deposits from 46 invasive ductal carcinomas of the breast were evaluated histologically and immunohistologically. The study yielded the following results: (1) tumor-infiltrating lymphoreticular cells preferentially accumulated in the stromal bands; the tumor foci generally showed a considerably lower degree of infiltration; (2) in most cases, monocytes/macrophages (Mono 1+) represented the overwhelming majority of tumor-infiltrating cells; (3) next in frequency were T-lymphocytes (Leu-1+), especially CD4+ lymphocytes (Leu-3a+), while CD8+ lymphocytes (Leu-2a+) mostly occurred only in moderate numbers; (4) B-lymphocytes (To15+), plasma cells, natural killer cells (Leu-7+), tissue mast cells, and T-accessory reticulum cells (OKT 6+) were observed mostly in low or very low numbers, while eosinophils were nearly absent and B-accessory reticulum cells (Ki-M4+) were totally absent from the lymphoreticular infiltrates. Definite conclusions regarding the functional properties of the tumor-infiltrating cells cannot be drawn from an immunohistologic analysis in situ alone, but the preferred localization of most tumor-infiltrating cells in the stroma does not support an intensive interaction between the host defenses and the metastatic tumor.

The influence of major histocompatibility complex class I antigens on tumor growth and metastasis

The work described here demonstrates the importance of major histocompatibility complex class I antigens for the control of tumor growth and metastasis by the host's immune system. In certain murine tumor cells which have lost expression of H-2 class I antigens, a de novo expression of H-2 can be achieved by transfection with syngeneic class I genes. In contrast to the parental cells the transfected tumors do not grow any more in syngeneic mice, or in other cases they do not form metastases. The studies suggest that the de novo expression of the H-2 antigens renders the tumors highly immunogenic and leads to effective recognition of a tumor-associated antigen in conjunction with the transfected H-2 antigen. These conclusions were confirmed in other tumor systems. For example, separation of a heterogeneous tumor into clones expressing high or low amounts of H-2 showed that only the tumor cell with low H-2 grew well in syngeneic mice, whereas the H-2 high tumor clones were rejected. In other studies in vitro induction by IFN-gamma of H-2 antigen on H-2 negative tumors led to reduced tumor growth in vivo which was due to the increased immunogenicity. About 10% of human tumors are also low or defective for HLA class I expression and often these tumors appear to be more malignant. The class I negative tumors could either have arisen from class I low or negative tissues or are HLA loss variants which escaped the attack of the immune system. Altogether, our studies and the data of other laboratories demonstrate the important role of class I antigens for anti-tumor immunity and they suggest that modulation of class I expression by gene transfection or by induction with soluble mediators could be a useful tool for the manipulation of tumor immunity.

Immunohistochemical analysis of HLA antigens and mononuclear infiltrates of benign and malignant breast

Using immunohistochemistry, the epithelial expression of HLA Class I and II antigens (and beta 2 microglobulin) was compared in benign and malignant breast, and the stromal lympho-histiocytic infiltrate of these tissues quantified. The findings were compared with certain characteristics of the carcinomas. In contrast to other studies, malignancy was found to be associated with a far greater infiltrate of both lymphocytes and macrophages. In carcinomas, lymphocyte but not macrophage numbers showed a positive correlation with epithelial HLA ABC and HLA DR expression. This was particularly striking for T lymphocytes, and especially for the T4 subset. The histological grade of carcinoma is closely related to the degree of HLA ABC and beta 2 microglobulin expression, but no such relationship was found for HLA DR. There were greater numbers of mononuclear cells in the poorer differentiated tumours compared with other grades. Those carcinomas with evidence of lymph node metastasis contained greater numbers of macrophages identified by the antibody Y1/82A, but none of the other parameters studied was found to be associated with local lymph node status. The use of a large panel of monoclonal antibodies against various cellular subtypes has allowed a more detailed analysis of the interaction between breast epithelium and the host response.

Immunohistological analysis of lymphocyte subpopulations infiltrating breast carcinomas and benign lesions

We characterized different subpopulations of infiltrating mononuclear cells using 8 monoclonal antibodies (MAbs) on serial cryosections of breast tissue from 85 cancer patients and 32 samples of benign lesions, and the ABC technique. In general, lymphocytes were found more frequently and more abundantly in cancerous lesions. The infiltrates consisted mainly of T-cells in close contact with malignant cell-nests. T-helper/inducer cells clearly predominated over T-suppressor/cytotoxic cells in neoplastic tissues, whereas in benign tissues the T-helper/suppressor ratios seemed to be well balanced. While a few MAb Leu-7 (HNK-I)-reactive NK cells were found in the stroma of the breast tumors, none could be identified in the noncancerous lesions. The correlation of these data with histology and tumor stage of patients has been evaluated by a quantitative approach using planimetry in an interactive registration system.

An immunohistologic study of the distribution and status of activation of head and neck tumor infiltrating leukocytes

We examined tumor infiltrating leukocytes (TIL) in frozen sections of 28 biopsies from squamous cell carcinomas of the head and neck (SCCHN). In so doing, we used monoclonal antibodies (MoAb) directed against various leukocyte antigens. As defined by HLe-1+ cells, leukocyte infiltration was present in all biopsies. The amount of HLe-1+ cells was more often greater in stage III than in stage IV lesions. Most of the TIL were identified as CD5+ T-lymphocytes. In contrast, CD19+ B-cells were sparse in most biopsies. CD14+ monocytes/macrophages were found in only a few specimens. The relative proportion of CD4+ T-helper cells was higher than or at least equal to CD8+ suppressor/cytotoxic cells in all samples tested. Interleukin-2 (IL-2) receptor+ lymphocytes were evident in 13 of 22 biopsies stained for CD25 reactivity, and were more often observed in stage III than in stage IV tumors. All biopsies from recurrent tumors had no detectable IL-2 receptor+ cells. Our findings provide evidence for a positive correlation between a greater amount of TIL in earlier stages of SCCHN. The presence of IL-2+ lymphocytes suggests that SCCHN may be capable of activating resting lymphocytes for further IL-2-induced proliferation.

In situ quantitation of inflammatory mononuclear cells in ductal infiltrating breast carcinoma. Relation to prognostic parameters

The authors examined inflammatory mononuclear cells in 10 fibroadenomas and 56 ductal infiltrating type carcinomas of the breast to see whether the distribution of various subpopulations of the mononuclear cells were correlated with known histologic, biochemical, and clinical parameters of the cancers. T cells, B cells, natural killer cells, and macrophages were quantitated on frozen tissue sections, which were stained with monoclonal antibodies, as demonstrated by the immunoperoxidase technique. The carcinomas had significantly higher numbers of T cells, Leu-3+ helper-inducer cells, T8+ cytotoxic-suppressor cells, M5+ macrophages, and T6+ Langerhans cells than the fibroadenomas. There were no significant differences in the distribution of natural killer cells. Among these mononuclear subsets, helper-inducer T cells were the primary cell type in both benign and malignant neoplasms. Estrogen receptor-positive carcinoma had significantly fewer numbers of T cells and the subgroup of Leu-3+ helper-inducer cells. Clinical Stage 3 cancers had significantly fewer numbers of T cells and natural killer cells than both fibroadenomas and Stage I and II neoplasms. Prominent infiltration of T cells, specifically helper-inducer cells, is not associated with estrogen receptor positivity, which is known to be a favorable prognostic marker of breast carcinoma. The findings of fewer T cells and natural killer cells in Stage 3 carcinomas are intriguing but unexplained.

Immunohistochemical characterization of seminoma and its inflammatory cell infiltrate

Monoclonal antibodies and the avidin-biotin immunoperoxidase technique were used to study the expression of major histocompatibility complex antigens and the nature of the inflammatory cell infiltrate in 10 testicular seminomas. Tumor cells did not react with anti-HLA-A,B,C, anti-HLA-DR, anti-HLA-DQ, and anti-beta 2 microglobulin antibodies to major histocompatibility antigens. All of the 10 tumors contained a slight to marked inflammatory cell infiltrate at the periphery of the tumor, in the connective tissue septa, and in the tumor lobules. The lymphocytes were predominantly T cells; B lymphocytes were rare. The tissue available for study from seven tumors showed tumor lobules separated by delicate fibrovascular septa; T lymphocytes with a cytotoxic-suppressor phenotype predominated in this area in six tumors. In the four tumors in which peripheral tissue was available for study, cells with a helper-inducer phenotype predominated at the tumor margin. Tissue from three tumors showed stromal sclerosis and a dense lymphohistiocytic infiltrate separating individual and small nests of tumor cells; T cells with a helper-inducer phenotype predominated in these cases. Aggregates of macrophages that expressed OKM-1 and Leu-M3 were present in eight of 10 tumors. These findings indicate that two types of immune reactions may be operating: a delayed-type hypersensitivity response at the periphery and a cytotoxic-suppressor effector mechanism in the tumor lobules. Furthermore, major histocompatibility complex antigens are not involved in eliciting the inflammatory response.

Immunohistochemical study on the distribution and significance of mononuclear cells in human breast carcinoma

Lymphocyte subsets in the tumor nests of breast carcinoma were immunohistochemically investigated and a quantitative analysis was added. The majority of cases showed predominance of T cell and suppressor T cell (T8). A decrease in number of lymphocyte subsets and the helper T (T4)/T8 ratio in the stroma of tumor nests correlated well with the progression of clinical stage and the presence of metastasis. This correlation could not be found in the peripheral region of the tumor nests. Macrophages and NK cells were infrequently observed only in the peripheral region of ductal carcinoma. T cell infiltration was prominent in medullary carcinoma with lymphocyte infiltration (MC), and macrophages, NK cells, and T zone histiocytes were frequently encountered. For the purpose of knowing the activity of T cells, IL-2 receptor (Tac) and transferrin receptor were examined immunohistochemically. The fact that a few activated T cells were found only in the peripheral region of tumor nest suggested the local immune response in ductal carcinoma not to be so active as to reject the tumor cells. Since numerous activated T cells were recognized in the tumor nests of MC, this type of breast carcinoma was thought to have a higher immune reactivity. There was little evidence indicating NK cells to play a role for natural cytotoxicity in breast carcinoma.

Inflammatory infiltrates of experimental mammary cancers

The purpose of this review was to summarize observations on the type and function of inflammatory infiltrates of mouse mammary tumors and to speculate on the underlying mechanisms and the significance of infiltrates to mammary tumor biology. Although the major conclusion is that much more work is needed, certain themes seem to be emerging. The number of infiltrating cells can be very high but is unrelated to biological behavior of the tumors. What seems to be important is the relative contributions of inflammatory cell subsets. In the case of T-cell subsets and NK cells, the infiltrates from tumors of long-term cell lines so far seem uninformative. The general characteristics are similar to those of infiltrates from rapidly proliferating, normal mammary tissues. These characteristics do not correlate with diverse biological behavior or malignant potential. A more informative model appears to be one in which the development of tumors from preneoplastic tissue can be observed. Here our attention is currently focused on NK cells. By contrast, the correlation between activated TAM and metastatic behavior suggests that our transplantable MMT lines may be biologically relevant in the study of infiltrating macrophages. We are especially interested in the role of TAM in the generation of tumor cell variability. Overall, our data indicate that the host infiltrate is another manifestation of both inter- and intra-tumor heterogeneity and, as such, is not simply a response to, but, rather, a part of the tumor ecosystem. Unraveling the cellular and molecular mechanisms that govern the inflammatory cell component of tumors should provide insight into the types of cellular interactions that result in tumor development and progression.

Separation, phenotyping and limiting dilution analysis of T-lymphocytes infiltrating human solid tumors

Tumor-infiltrating lymphocytes (TIL) were obtained by a combination of mechanical release and enzymatic disaggregation from 35 human solid tumors. The number of lymphocytes in TIL-enriched suspensions varied from 1 X 10(4) to 7.6 X 10(6) per wet gram of tumor. The TIL preparations separated by differential centrifugation on Ficoll-Hypaque gradients contained 10-95% of T11+ cells (mean 50%), and tumor cells accounted for the other major cellular component. Macrophages, NK cells, B cells and granulocytes were infrequently seen. Morphologically, TIL-T were small non-activated cells. They expressed the T11 and T3 antigens but not the receptor for IL-2 (IL-2R) or HLA-DR antigens as determined by double immunofluorescence staining. Rare T11+/IL-2R+ cells were recovered only from colon and lung carcinomas. The mean T4/T8 ratio in 12 TIL preparations was 1.1 +/- 0.8. Immunohistology with monoclonal antibodies (MAbs) performed in 31/35 tumors confirmed that the T11+ cells infiltrating solid tumors rarely expressed the IL-2R and that the cell content of suspensions enriched in TIL was comparable to that determined in situ. The recovered TIL were cloned in a microculture system that permits proliferation of nearly all normal peripheral blood T lymphocytes (PBL-T). Under these culture conditions, frequencies of the proliferating T lymphocyte precursors (PTL-P) were depressed in both the TIL preparations (less than 0.01 to 0.39) and patients' PBL-T (0.05 to 0.5). These low frequencies of PTL-P were seen in patients with all tumor types, both primary and metastatic.

Monoclonal antibody CI-panHu defines a pan-human cell-surface antigen unique to higher primates

The murine monoclonal antibody CI-panHu reacts strongly with the cell surface of all human cells, including erythrocytes, tumour cells and HLA-A,B,C-negative cell lines. As such, this antibody defines the first pan-human cell-surface antigen reported. The antigenic determinant detected is associated with a protein doublet of 16,000 MW whose expression is restricted to cells from humans, apes and some species of Old World monkeys. Antibody reactivity is not diminished by routine fixation procedures, nor by paraffin-embedding, and the antigenic determinant is relatively protease-resistant. The use of this antibody as a positive control in immunoassays of human cells is discussed.

Loss of HLA-A,B,C and de novo expression of HLA-D in colorectal cancer

One hundred primary colorectal carcinomas and 19 metastases were studied for the expression of HLA-A,B,C and HLA-D antigens using monoclonal antibodies (MAbs) against framework determinants and a sensitive immunoperoxidase technique on frozen sections. With MAb W6/32, 65 tumors were intensely stained for HLA-A,B,C; 22 showed a reduced staining intensity, in 5 carcinomas a minor or major subset of the tumor cells lacked HLA-A,B,C, and in 8 cases all tumor cells were unreactive. The loss of HLA-A,B,C was inversely correlated with the degree of differentiation. No relation was found to type, stage and site of the tumor, or to age and sex of the patients. With MAb 2.06, 58 tumors were HLA-D negative, 20 exhibited small positive foci, 19 showed a patchy staining pattern, and 3 were homogeneously HLA-D positive. There was no apparent correlation between HLA-D expression and any of the clinico-pathological features mentioned, nor was it related to the loss of HLA-A,B,C. About half of the metastases displayed the same staining pattern for HLA-A,B,C and -D as the respective primary tumors. The remainder differed from the primary carcinoma by gain or loss of HLA antigens.

Lymphoreticular infiltrates in invasive ductal breast cancer. A histological and immunohistological study

Fifty-two invasive ductal breast cancers were investigated histologically and immunohistologically to assess localization and composition of the lymphoreticular infiltrates. The tumour-infiltrating cells were mainly located in the intervening stroma, whereas tumour foci often exhibited lower numbers of lymphoreticular cells. Macrophages (Mono 1+ and KiM 6+) and helper/inducer cells bearing the T4 surface antigen (Leu-3a+) regularly constituted the majority of the tumour-infiltrating lymphoreticular cells. In more than 80% of cases large numbers of macrophages were found, and many T4 cells occurred in about 60%. Next in frequency were the T lymphocytes (Leu-1+) which were mostly observed in high (46%), or in moderate (39%) numbers. In about 2/3 of the cases moderate numbers of T8 (suppressor/cytotoxic) lymphocytes (Leu-2a+) were detected. B lymphocytes (T0 15+) and natural killer cells (Leu-7+) were generally encountered in very low numbers, while eosinophilic granulocytes were virtually absent from the lymphoreticular infiltrates. Tissue mast cells and plasma cells were present in very low numbers in about one half of the tumours but cases with low, moderate or - rarely - even high numbers of infiltrating cells also occurred. It must be emphasized that an in situ histomorphological analysis of the cellular part of the stromal reaction of invasive ductal breast cancers allows only limited conclusions concerning the functional properties of the tumour-infiltrating lymphoreticular cells. From the present study, macrophages and T4 cells but also T8 lymphocytes might be of significance in immunooncological reactions "against" clinically detectable stages of invasive breast cancer.

Clonal analysis and in situ characterization of lymphocytes infiltrating human breast carcinomas

T lymphocytes were isolated from tumor biopsies in 13 patients with breast carcinomas. Immunohistology with monoclonal antibodies confirmed the presence of mononuclear cell infiltrates composed primarily of T lymphocytes in all tumors studied. While the proportion of T lymphocytes expressing the T4 or the T8 surface marker varied from tumor to tumor as determined by morphometric analysis, T8+ cells were more numerous than T4+ cells in 8/12 breast tumors studied. Relatively few T cells (less than 10% in 11/12 tumors) were in an activated state as judged by the surface expression of HLA-DR antigens or the receptor for interleukin-2 (IL-2). In 1 case 20% of the infiltrating mononuclear cells were expressing the IL-2 receptor. The tumor infiltrating lymphocytes (TIL) recovered from 10 tumors were cloned in a microculture system that permits proliferation of nearly 100% of normal peripheral blood T lymphocytes (PBL-T). In contrast to normal and autologous PBL-T, frequencies of proliferating T lymphocyte precursors (PTL-P) were depressed (less than 0.01) in 7/10 TIL preparations indicating a decreased responsiveness of TIL to phytohemagglutinin at the single-cell level. The frequency of PTL-P was noticeably higher in 2 cases (0.03 and 0.09) and close to normal in 1 case (0.39). A total of 170 clones were expanded in vitro and analyzed for different functional capabilities. Most of these clones expressed the T4+/T8-phenotype (73%) and strikingly 53% of these T4+/T8- clones were cytolytic in a lectin-dependent assay, a functional subset which is uncommon among normal PBL-T. Some clones (10%) lysed allogeneic breast tumor cells (MCF7). Only 15% of the clones displayed natural killer activity. Among the cytolytic clones, 17 of 31 tested were also IL-2 producers irrespective of the T4 or T8 phenotype. Our results show that human mammary carcinomas contain many infiltrating T cells with cytolytic potential. Interestingly, among the proliferating cytolytic T cell clones (56% of the microcultures), many expressed the T4+/T8- phenotype. These findings may indicate that the in situ cytolytic reaction (against unknown antigens) is associated preferentially with class II antigens.