Anti-major Histocompatibility Complex Class Research Articles

Isolation of a new superantigen with potent mitogenic activity to murine T cells from Streptococcus pyogenes

A mitogenic substance on murine lymphocytes was detected in the culture supernate of Streptococcus pyogenes type 12 strain. This substance had a molecular weight of 28 000 and p I 9.2, and was designated as S. pyogenes mitogen (SPM). The proliferative response of C3H HeN spleen cells began at 1 ng ml −1 and reached a maximal response at 100 ng ml −1 of SPM for 4 days culture. Anti-Thy 1.2 mAb and complement-treated spleen cells abrogated the proliferative response to any dose of SPM. Although the anti-major histocompatibility complex class I mAbs had no blocking effect on proliferation by SPM, this proliferation was substantially inhibited by the addition of either anti-I-A or anti-I-E mAb, and complete inhibition was produced by the addition of both mAbs. Fixed antigen-presenting cells still induced T cell proliferation by SPM. A significant expansion of T cells bearing Vβ13 T-cell receptor was observed up to 73% among the Thy1.2 + cells in cultures stimulated with SPM, indicating expansion in a Vβ-specific manner. Immunoblotting of IEF-separated proteins showed that anti-streptococcal pyrogenic exotoxin (SPE) C reacted with a protein of p I 6.9 and anti-SPEB did not show any reactivity. SPEA was reported to expand Vβ8.1 and 8.2 bearing murine T cells, and SPM did not. SPM also exhibited potent mitogenic activity on human T cells and Vβ21 + T cells were selectively expanded. These results lead to the conclusion that SPM was neither SPEA, B nor C, but a new protein belonging to a group of streptococcal superantigens with activity on not only human but also murine lymphocytes.

Isolation of a new superantigen with potent mitogenic activity to murine T cells from Streptococcus pyogenes.

A mitogenic substance on murine lymphocytes was detected in the culture supernate of Streptococcus pyogenes type 12 strain. This substance had a molecular weight of 28,000 and pI 9.2, and was designated as S. pyogenes mitogen (SPM). The proliferative response of C3H/HeN spleen cells began at 1 ng ml-1 and reached a maximal response at 100 ng ml-1 of SPM for 4 days culture. Anti-Thy 1.2 mAb and complement-treated spleen cells abrogated the proliferative response to any dose of SPM. Although the anti-major histocompatibility complex class 1 mAbs had no blocking effect on proliferation by SPM, this proliferation was substantially inhibited by the addition of either anti-I-A or anti-I-E mAb, and complete inhibition was produced by the addition of both mAbs. Fixed antigen-presenting cells still induced T cell proliferation by SPM. A significant expansion of T cells bearing V beta 13 T-cell receptor was observed up to 73% among the Thy 1.2+ cells in cultures stimulated with SPM, indicating expansion in a V beta-specific manner. Immunoblotting of IEF-separated proteins showed that anti-streptococcal pyrogenic exotoxin (SPE) C reacted with a protein of pI 6.9 and anti-SPEB did not show any reactivity. SPEA was reported to expand V beta 8.1 and 8.2 bearing murine T cells, and SPM did not. SPM also exhibited potent mitogenic activity on human T cells and V beta 21+ T cells were selectively expanded. These results lead to the conclusion that SPM was neither SPEA, B nor C, but a new protein belonging to a group of streptococcal superantigens with activity on not only human but also murine lymphocytes.

Chronic experimental Chagas' disease: functional syngeneic T-B-cell cooperation in vitro in the absence of an exogenous stimulus.

We have investigated CD4+ T-cell autoreactivity to normal syngeneic B cells in vitro in chronic experimental Chagas' disease. Resting B cells induced an intense proliferative response and lymphokine secretion by splenic CD4+ T cells from Trypanosoma cruzi-infected (8 months or more of infection) donors, compared to much lower responses by uninfected controls. On the other hand, lipopolysaccharide-activated B cells induced syngeneic CD4+ T-cell activation in both control and infected groups. The observed syngeneic T-B-cell cooperation was bidirectional. In the absence of any exogenous stimulus, CD4+ T cells from T. cruzi-infected animals induced much higher production of all tested immunoglobulin (Ig) isotypes (IgM, IgG1, IgG2a, IgG2b, IgG3) by syngeneic B cells, compared to T cells from uninfected donors. When lipopolysaccharide-treated B cells were used, CD4+ T cells from either control or infected donors enhanced IgG1 and IgG3 production, but only CD4+ T cells of infected origin induced IgG2a production in this system without addition of exogenous gamma interferon. Enhanced T-cell proliferation and Ig production were also observed with highly purified CD4+ T cells and in serum-free medium. Both proliferation and Ig production could be blocked with anti-major histocompatibility complex class II monoclonal antibodies. Enhanced reactivity and help for Ig production were seen only in response to syngeneic BALB B cells and not in response to allogeneic B10 B cells. These results indicate that chronic infection with T. cruzi results in increased CD4+ T-cell reactivity towards syngeneic B cells, which leads to spontaneous Ig production. These autoreactive T cells might play a role in polyclonal autoantibody production in chronic Chagas' disease.

5435999 Proliferative action of leukemia inhibitory factor on satellite cells

To increase understanding of structural and phenotypic characteristics of dendritic cells (DCs) in the periapical region of the periodontal ligament (PDL) of rat molars, we performed immunoelectron microscopy for CD11c, a previously untested DC marker. Results demonstrated that CD11c clearly recognized DCs, although it also labeled certain macrophage subpopulation(s). In the normal PDL, resident DCs that extended several long cytoplasmic processes from their oval to slender cell body were identified. When the PDL was bacterially challenged by three days of unsealed pulp exposure, relatively small, irregularly shaped DCs with many thin and short cytoplasmic processes, most likely representing newly recruited DCs, were detected. The ratio of DCs in CD11c+ cells was significantly higher than that in ED1 (anti-macrophages and DCs)-positive and OX6 (anti-major histocompatibility complex class II molecules)-positive cells in normal and challenged PDL.

Generation of Human Dendritic Cells/Langerhans Cells From Circulating CD34+ Hematopoietic Progenitor Cells

Human Langerhans cells (LC) are CD1a+ dendritic cells (DC) that function as potent antigen-presenting cells for primary and secondary immune responses. Limitations in DC/LC numbers, imposed by difficult and tedious isolation procedures, have so far precluded their use as immunogens in the generation and/or augmentation of host responses against various pathogens. Therefore, we have developed a procedure for the generation of human DC/LC from CD34+ hematopoietic progenitor cells (HPC) isolated (mean: 0.7 x 10(6)/ buffy coat and 2.6 x 10(6)/leukapheresis product) and purified ( > 95%) from the peripheral blood of healthy adults. In vitro stimulation of these cells with granulocyte-macrophage colony-stimulating factor (GM-CSF) and tumor necrosis factor (TNF)-alpha led to their vigorous proliferation and differentiation resulting in the emergence of CD45+/CD68+/CD3-/CD19-/CD56- leukocytes some of which (mean: 12%) express CD1a and exhibit anti-CD4 and anti-major histocompatibility complex (MHC) class II reactivity. These CD1a- leukocytes include (1) LC as evidenced by the presence of Birbeck granules (BG), (2) CD14+ monocytes, and (3) Birbeck granule-negative cells with a dendritic morphology. Addition of interleukin (IL)-4 to the cytokine cocktail interfered with the development of monocytes and led to a reduction in the overall yield but, on the other hand, resulted in an increased percentage of CD1a+ cells (mean: 24%) among all cells generated. In vitro generated CD1a+, but not CD1a- HPC-derived cells are potent stimulators of the primary mixed leukocyte reaction and, as such, promising candidates for vaccination purposes.

Staphylococcal Enterotoxin B Upregulates Expression of ICAM-1 Molecules on IFN-γ-Treated Keratinocytes and Keratinocyte Cell Lines

The effects of staphylococcal enterotoxin B (SEB), a Staphylococcus aureus-derived bacterial superantigen, on expression of intercellular adhesion molecule-1 (ICAM-1) were examined in cultured normal and transformed (DJM-1 cells) human keratinocytes by flow cytometry, confocal microscopy, digital image processing, and reverse transcriptase-polymerase chain reaction. SEB significantly upregulated ICAM-1 expression in the interferon-gamma (IFN-gamma)-pretreated, HLA-DR-positive normal keratinocytes and DJM-1 cells in a dose-dependent manner, but not in the untreated, HLA-DR-negative cells. Other toxins such as diphtheria and pertussis toxins did not have the effect. The distribution of SEB and HLA-DR molecules was identical on the IFN-gamma-treated, HLA-DR-positive DJM-1 cells by confocal microscopy. Digital image processing analysis demonstrated that SEB induced a transient increase of intracellular calcium concentration only in the IFN-gamma-treated DJM-1 cells. Pretreatment of the IFN-gamma-treated DJM-1 cells with anti-major histocompatibility complex class II monoclonal antibody completely blocked the effect of SEB. Furthermore, ICAM-1 mRNA was detected in the IFN-gamma-pretreated, SEB-exposed normal keratinocytes by reverse transcriptase-polymerase chain reaction. Our results demonstrate that SEB binds to keratinocytes, presumably via major histocompatibility complex class II molecules such as HLA-DR, triggers calcium mobilization, and induces the synthesis of ICAM-1 molecules. We speculate that, in various cutaneous disorders, SEB penetrates the epidermis and interacts with HLA-DR-positive keratinocytes to upregulate ICAM-1 expression, thus modulating the course of the inflammatory process.

Discrimination between major histocompatibility complex class II DQ and DR locus products in cattle.

We have used a panel of anti-major histocompatibility complex (MHC) class II monoclonal antibodies (mAbs) and have assessed their specificity for the products of the individual bovine MHC (BoLA) class II subregions. The mAbs identified two distinct class II molecules by affinity purification and ELISA. Two-dimensional immunoblotting confirmed these data and NH2-terminal sequencing of the purified class II alpha chains of one member of each group identified the subregion specificity of the mAbs. The mAbs VPM36, TH22A and TH81A are specific for BoLA DQ, whereas VPM54, TH14B and J11 are specific for BoLA DR. SW73.2 reacts with both MHC subgroups of all cattle tested.

Target cells of cytotoxic T lymphocytes directed to the individual structural proteins of rabies virus.

Target cells of cytotoxic T lymphocytes (CTL) directed to the individual structural proteins (except for the large polymerase (L) protein) of rabies virus were established by expressing only the respective protein in murine neuroblastoma (NA) and murine macrophage (J774-1) cell lines. Mice infected with the ERA strain of rabies virus developed CTL responses to all of these rabies virus proteins. The cytotoxic activity was abrogated by pretreatment of the effector cells with anti-CD8 monoclonal antibody (MAb) and complement but not with anti-CD4 MAb. Cell lysis by CTL was blocked in the presence of anti-major histocompatibility complex (MHC) class 1 antibodies in J774-1 cell lines. Rabies virus-infected cells express these proteins at the surface, which can be recognized and lysed by the respective CTL. Mice immunized with beta-propiolactone-inactivated virus induced a CTL response against glycoprotein but not against internal viral components. This assay system might be useful for further analysis of the possible contribution of these proteins in the cell-mediated immune protection against rabies.

Role of lymphocytes in spontaneous regression of experimentally transplanted canine transmissible venereal sarcoma.

Sensitized peripheral blood lymphocytes (PBL) obtained from canine transmissible venereal sarcoma (CTVS)-regressed dogs were more cytotoxic against CTVS cells than non-sensitized PBL from untreated dogs. Cytotoxicity shown by sensitized PBL was inhibited significantly by the addition of anti-major histocompatibility complex (MHC) class II mouse monoclonal antibody as well as that of anti-dog thymocyte rabbit serum. The degree of cytotoxic activity shown by lymphokine activated killer (LAK) cells induced from non-sensitized or sensitized PBL was similar to that of the activity shown by sensitized PBL. These LAK activities were also prohibited by the addition of anti-dog thymocyte rabbit serum. Immunohistochemical examination demonstrated that MHC class II antigens were expressed on the surface membrane of CTVS cells and thymocyte antigens were detected on the surface of the tumor infiltrating lymphocytes. From the results mentioned above, lymphocytes which play a central role in tumor regression are considered to be T cells. These cells might recognize MHC class II antigens on the surface membrane of CTVS cells in tumor regression.

Immunological responses against an autologous human hepatocellular carcinoma cell line.

We performed a detailed analysis of immune responses in a hepatocellular carcinoma (HCC) cell line and effector cells obtained from a patient with HCC. We examined the cytotoxic activity of natural killer (NK) cells, lymphokine-activated killer (LAK) cells and cytotoxic T lymphocytes (CTL) against an autologous tumour cell line (SUHC-1) to investigate the immune mechanism of human lymphocytes against HCC cells. Cytotoxic T lymphocytes were induced by co-culturing of peripheral blood lymphocytes (PBL) and SUHC-1 cells, mixed lymphocyte and tumour cell culture (MLTC). The susceptibility of SUHC-1 to NK and LAK cells was similar to that of other allogeneic cell lines, such as K562, PLC/PRF/5 and Mahlavu. Effector cells induced in the primary MLTC had high cytotoxic activity but were not specific for SUHC-1. Cytotoxic T lymphocytes with specific activity against SUHC-1 were induced after PBL were stimulated five times at 7-10 day intervals with SUHC-1 and low-dose recombinant interleukin-2 (rIL-2), suggesting that as the culture progressed, broadly reactive effector cells disappeared and specific effector cells survived. The specific effector cells were identified as CD3+/CD4+ and CD3+/CD8+ T-lymphocyte subsets. The recognition mechanisms of CD3+/CD4+ CTL remain unresolved because the cytotoxicities were not inhibited by anti-CD4 and anti-major histocompatibility complex (MHC) class II monoclonal antibodies (MoAb). Treatment of cells with anti-CD3, anti-CD8 and anti-MHC class I MoAb partially inhibited lysis. These results demonstrated that the T-cell receptor (TCR)/CD3 complex appeared to be involved in SUHC-1 specific antigen recognition and antigen recognition of CD3+/CD8+ CTL was MHC class I restricted.

Tumor-Infiltrating Lymphocytes Derived From Select B-Cell Lymphomas Secrete Granulocyte-Macrophage Colony-Stimulating Factor and Tumor Necrosis Factor-α in Response to Autologous Tumor Stimulation

Tumor infiltrating lymphocytes (TIL) were cultured from 17 B-cell lymphoma specimens derived from patients with predominantly low-grade malignancies. Specimens included 15 lymph-node biopsies, 1 malignant pleural effusion, and PBL from 1 patient with circulating lymphoma cells. The phenotypic and proliferative characteristics of TIL cultured in interleukin-2 (IL-2) were studied, as well as cytolysis and cytokine secretion in response to autologous tumor. Flow cytometry of fresh tumor suspensions showed that 50% of cells (median) were malignant B cells and 36% were infiltrating T lymphocytes. After culture for approximately 1 month, TIL were 75% +/- 8% CD3+ (mean +/- SEM), 47% +/- 8% CD4+ and 35% +/- 7% CD8+. TIL proliferation was modest in most cases: the median maximum expansion was 32-fold in 25 days. Lysis of autologous tumor in 4-hour 51Cr release assays was mediated by 2 of 12 TIL studied, but was nonspecific. However, these same two TIL, when cocultured with various tumor stimulators, preferentially secreted tumor necrosis factor-alpha and granulocyte-macrophage colony-stimulating factor after autologous tumor stimulation; unstimulated TIL secreted undetectable or barely detectable levels of these cytokines. In one TIL culture, cytokines were secreted by purified CD4+ TIL but not by CD8+ cells, and secretion was completely abrogated by the anti-major histocompatibility complex (MHC) class II antibody IVA12. Thus, although specific cytokine secretion by lymphoma TIL in response to autologous tumor was observed, it occurred in fewer than 20% of patients studied.

Effective Enrichment of Murine Epidermal Langerhans Cells by a Modified (Mismatched) Panning Technique

A method for the enrichment of murine epidermal Langerhans cells (LC) is described in detail. It is based on positive selection of LC from pre-enriched fresh or cultured epidermal cell suspensions derived from ear skin by a modified panning technique. The method uses the interspecies cross-reactivities of anti-immunoglobulin antibodies: when LC in an epidermal cell suspension are labeled with mouse anti-major histocompatibility complex (MHC) class II antibodies they bind to petri dishes coated with anti-rat immunoglobulin antibodies. We therefore call this method "mismatched panning." After rinsing off non-adherent cells, the adherent LC can easily be dislodged by adding excess amounts of rat immunoglobulins, which effectively compete with the LC-bound mouse anti-MHC class II antibodies for binding to the petri dish. Using this modified panning technique, both fresh and cultured LC could be enriched up to more than 90% purity. From one ear, 2.0-3.0 x 10(4) fresh LC and 3.0-4.5 x 10(4) cultured LC could be obtained. Of all LC present in a primary, unenriched epidermal cell suspension, 40-60% were recovered when panned immediately after isolation of the epidermal cells and 50-75% when panned after 3 d of epidermal cell culture. Viability of panned LC was consistently more than 90%. Antigen presenting and T-cell-stimulating capacity of LC and responses to the cytokines granulocyte/macrophage colony-stimulating factor and tumor necrosis factor-alpha were not impaired by this panning procedure. The major advantage of this method compared to pre-existing panning techniques is the ease with which adherent LC can be dislodged from the panning dishes. Because the elution procedure is very gentle, virtually all panned LC are viable. As a consequence, good yields of highly enriched LC can be obtained in a reasonable time.

Production and characterization of two new mouse monoclonal antibodies reactive with denatured mouse class II beta chains.

We report on the production and characterization of two murine peptide specific anti-major histocompatibility complex class II chain specific monoclonal antibodies. Two new mouse monoclonal antibodies reactive with two synthetic peptides corresponding to Abb amino acids (146-157) and Abs amino acids (146-157) were produced. KL295 is the mouse anti-Abb monoclonal antibody, which reacts with denatured beta chains of H-2b, H-2d, H-2p, and H-2q, but fail to react with spleen cell lysates from H-2f, H-2j, H-2k and H-2s mice. The mouse anti Abs monoclonal antibody, KL304 in contrast reacts with the denatured Class II beta chains of H-2f, H-2j, H-2k and H-2s mice, but fails to bind H-2b, H-2d, H-2p or H-2q beta chain, suggesting residues 153-155 of this molecule to be critical for the epitope. Both KL295 and KL304 bind B10.WB (H-2j) which suggests a unique epitope in this strain of mice. Neither KL295 or KL304 react with native mouse class II cell surface molecules.

Induction of antigen‐specific CD8+ cytolytic T cells by the exogenous bacterial antigen streptolysin O in rhesus monkeys

We have characterized the T cell responses induced by streptolysin O (SLO), a sulfhydryl-activated hemolysin secreted by streptococci, by applying long-term in vitro culture and cloning rhesus monkey (Macaca mulatta) T cells. T cell lines specific for SLO were obtained from three rhesus monkeys. These T cell lines required autologous antigen-presenting cells (APC) to proliferate in response to SLO and did not respond to purified protein derivative. Phenotypic analysis showed that the cells from two of three SLO-specific T cell lines were more than 85% CD3+CD4-CD8+ after prolonged in vitro culture. The rh 1842 CD8+ T cell line proliferative response to SLO was inhibited by the addition of anti-major histocompatibility complex (MHC) class I and anti-CD8 but not of anti-MHC class II and anti-CD4 monoclonal antibody (mAb). This cell line was able to lyse P815 target cells in the presence of anti-CD3 mAb and did not show natural killer activity. Moreover, specific lysis of autologous but not allogeneic non-rosetting E- cell targets pulsed with SLO was observed. Such lysis was inhibited by the addition of anti-MHC class I mAb. In the attempt to identify the restriction elements involved in SLO presentation APC from six unrelated rhesus monkeys and three humans were used. A CD4+ rh 1842 T cell clone responded when SLO was presented by one of six, and a CD8+ rh 1842 T cell clone by four of six rhesus monkeys APC. Both CD4+ and CD8+ T cell clones did not respond when SLO was presented by human APC. However, both clones responded when APC from all donors were used in conjunction with anti-CD3 mb. Furthermore, SLO required active processing to be presented to CD4+ and CD8+ T cell clones as glutaraldehyde fixation of APC before but not after antigen pulsing inhibited T cell proliferation. The SLO-specific CD8+ cytolytic T cells described here could play a role in the regulation of the immune response occurring during streptococcal infections and/or could participate in the pathogenesis of poststreptococcal nonsuppurative sequelae.

Production and characterization of a peptide specific, anti-major histocompatibility complex class II, monoclonal antibody

Monoclonal antibodies to major histocompatibility complex Class II proteins have been useful probes in understanding both the biochemistry and biology of these proteins. Almost all of the monoclonal antibodies previously described have been produced by immunization of mice with living cells. These antibodies react with native Class II proteins, but not usually with denatured material. It has been difficult to obtain specific anti-Class II antibodies which react with denatured proteins. Antibodies reactive with denatured proteins and with well-defined specificities would be useful in studies of Class II assembly and trafficking during the process of antigen presentation. In order to produce such an antibody we have immunized hamsters with a synthetic peptide corresponding to residues 146–177 (β 1 domain) of the mouse A b β protein. An antibody has been produced which reacts with the mouse Class II A β chain from H-2 b, H-2 d, H-2 p, and H-2 q mice in immunoblotting assays, but not with the β chain from H-2 f, H-2 j, H-2 k or H-2 s mice. Comparison of the amino acid sequences of these proteins along with the reactivity patterns of the antibody on synthetic peptides corresponding to homologous regions from A b β, A k β, A b α and D p suggest that the region of 153 to 155 is critical for the reactivity of this antibody. This antibody does not react with native Class II protein found on the surface of living mouse cells.

A quantitative analysis of T lymphocyte populations in human liver allografts undergoing rejection: The use of monoclonal antibodies and double immunolabeling

The aim of this study was to quantitate T-cell populations infiltrating portal tracts, bile ducts and hepatic lobules in 82 biopsy specimens from 25 patients after orthotopic liver transplantation. Biopsy specimens taken immediately after revascularization of the graft were used as controls. Patients studied include 18 with initial rejection episodes, 11 with unresolved rejection, five with vanishing bile duct syndrome and eight patients with other forms of liver injury. Quantitation was done in a blinded fashion for the first 20 biopsy specimens. A double immunolabeling technique was used to simultaneously immunolabel bile duct structures (with anti-major histocompatibility complex class II or antikeratins) and lymphoid populations (with anti-CD2, anti-CD4 or anti-CD8). This facilitated the accurate quantitation of intraepithelial lymphocytes within bile ducts. This technique also enabled simultaneous detection of CD4 and CD8 antigens on lymphocytes in portal tracts. The predominant lymphocyte subtype within biliary epithelium during acute and chronic rejection was of the CD2+/CD8+ phenotype. CD8+/CD4+ ratio in bile ducts was approximately 5:1 in acute, unresolved and chronic rejection. In vanishing bile duct syndrome, double immunolabeling enabled the detection of destroyed interlobular bile duct remnants that were not apparent on routine hematoxylin and eosin staining. Attached to some of these structures were CD8+ lymphocytes. Lobular CD8+ cells were not prominent in acute rejection but increased significantly in biopsy specimens from patients with unresolved and chronic rejection. In chronic rejection, a selective increase was seen in these CD8+ cells in centrizonal regions.(ABSTRACT TRUNCATED AT 250 WORDS)

T cell activation by a bispecific anti‐CD3/anti‐major histocompatibility complex class I antibody

A bispecific anti-CD3/anti-major histocompatibility complex (MHC) class I antibody (Ab) that is able to activate human T cells in the absence of a second signal has been used to compare activation by this Ab to signaling by anti-CD3 Ab or by antigen (Ag). We have studied early, intermediate and late events that occur after triggering of a tetanus toxoid-specific T cell clone. While bivalent anti-CD3 Ab induce transient (less than 20 min) intracellular Ca2+ concentration increases, the bispecific Ab, like antigen-presenting cells plus Ag, produces sustained (greater than 2 h) elevated intracellular Ca2+ concentration. In addition, while the anti-CD3 Ab only induces low levels of c-myc mRNA lasting less than 3 h, the bispecific Ab induces high levels which are maintained for at least 8 h after triggering. Late events, such as interleukin 2 receptor expression and proliferation, occurring more than 20 h after activation, were seen only when T cells were stimulated by bispecific Ab or by antigen-presenting cell and Ag. Cells activated by linking T cell receptor (TcR)/CD3 to MHC class I antigens could not be activated by cross-linking the MHC and TcR/CD3 structures separately. Therefore, we propose that the bispecific Ab functions by modifying the membrane dynamics of the TcR/CD3 complex, rather than by the generation of a second signal through class I antigens.

Naturally cytotoxic tonsillar leukocytes: phenotypic characterization of the effector population.

Human palatine tonsil sections were examined to investigate the distribution of cells bearing the cell surface markers of peripheral blood natural killer (PB-NK) cells. Leu-7+ (HNK-1+) cells were localized predominantly in lymphoid follicles, whereas OKM1-, Mac-1-, and Mo2-labelled cells were found in the epithelial and subepithelial regions and epithelial crypts. OKT10+ cells showed a variable distribution, being found in follicles and interfollicular or subepithelial regions. No. B73.1+ cells could be identified in tonsil sections. Leu-7+ cells appeared not to be responsible for tonsillar natural cytotoxicity, since Leu-7 (HNK-1) antibody- and complement-mediated lysis under conditions that markedly reduced PB-NK activity failed to abolish cytotoxicity, and positive selection by means of the FACS IV gave no enrichment of activity. Similarly, cells labelled with the antibodies B73.1, Leu-11b, OKT8, OKT10, and TDR 31.1 (anti-major histocompatibility complex class II framework determinant) were not enriched with regard to NK activity either. However, positive selection with OKM1, Mac-1, or Mo2 showed that cells bearing these markers were responsible for essentially all tonsillar NK activity. No large granular lymphocytes were identified in such populations enriched for NK activity. The observation that PB-NK cells labelled faintly with Mo2 weakens the argument that a non-adherent mononuclear phagocyte population was responsible for the activity. These data therefore support the existence of heterogeneity within naturally cytotoxic cell populations.