The paracortical area in dermatopathic lymphadenitis and other reactive conditions of the lymph node

We compared the paracortical area in 4 cases of dermatopathic lymphadenitis (DL) with the same area in 11 cases of various other reactive conditions of the lymph node by immuno- and enzymehistochemical techniques. In addition, electron microscopy was performed on three cases of DL. The paracortical nodules in DL proved to be composed of a variable number of dendritic, OKT6+ OKIa + ATPase+ cells, admixed with helper T-lymphocytes. All other lymph nodes studied lacked dendritic OKT6+ cells, whereas OKIa positivity was found in the cortical (follicular centers and mantle zones) and paracortical area (lymphocytes and scattered dendritic cells). Short incubation for ATPase revealed a paracortical, pericellular staining pattern in cases of DL, whereas in all other cases this staining pattern was observed only after long incubation times. On electron microscopy, three types of dendritic cells were found in DL, namely interdigitating reticulum cells ( IDRC ). Langerhans cells (LC) and macrophages. Intermediate forms between IDRC and LC, containing a few Birbeck granules and a well developed rough endoplasmic reticulum, were found. It is suggested that immunoreactivity for the monoclonal antibody OKT6 is restricted to cases of DL, and is due to the appearance of dendritic cells that have LC-characteristics. These cells either arrive from the skin along afferent lymph vessels, or are the result of a local transformation process of IDRC that acquire LC-characteristics, i.e. OKT6 immunoreactivity and Birbeck granules.     

The paracortical area in reactive lymph nodes demonstrating sinushistiocytosis

The enzyme and immunohistochemical features of lymphnodes showing sinus histiocytosis have been studied. Sinus histiocytes with phenotype OKM1+ OKT4+ Leu3a+ To5+ OKIal- showed strong acid phosphatase and non-specific esterase, weak endogenous peroxidase and no ATPase activities. In nine out of ten lymph nodes, paracortical collections of dendritic OKT6+ OKIal+ cells were observed. In two of the four cases studied these dendritic cells showed strong ATPase activity. We suggest that the dendritic OKT6+ OKIal+ ATPase+ interfollicular cells represent newly arrived veiled cells (VC) which have entered the lymph node by the afferent lymph, settled in the interfollicular area and are probably involved in the induction of a cellular immune response. OKT6+ OKIal+ ATPase+ VC may subsequently transform into mature, OKT6- OKIal+ ATPase+ interdigitating reticulum cells which are involved in the negative feedback of the cellular immune response. The association with sinus histiocytosis is probably related to the fact that an increase in mononuclear phagocytes in the afferent lymph is accompanied by a relative increase in VC. Our results demonstrate that in lymph nodes showing sinus histiocytosis, two cell types increase in number, i.e. an Ia- sinusoidal cell, engaged in phagocytosis of foreign material, and an Ia+ dendritic cell in the interfollicular area, probably involved in the induction of a cellular immune response.     

Analysis of lymphoid and dendritic cells in human lymph node, tonsil and spleen. A study using monoclonal and heterologous antibodies

The distribution of lymphoid and dendritic cells in human reactive lymph nodes, tonsils and spleens was examined by means of an indirect immunoperoxidase technique, using a panel of monoclonal and heterologous antibodies. The antibodies used were directed against antigens present on T cell subsets (Leu1, leu2a, Leu3a, TA1, OKT6), various types of B cells (BA1, BA2, HLA-DR, CR1) and cells of the mononuclear phagocyte system (alpha HM1, TA1, CR1, OKM1, NA 1/34). In the lymph node and tonsil Leu3a-positive cells (T-helper/inducer phenotype) and Leu2a-positive cells (T-suppressor/cytotoxic phenotype) are found in the thymus-dependent or T-cell area; in the spleen Leu3a-positive cells are found mostly in the periarteriolar lymphocyte sheath (PALS), while Leu2a-positive T-suppressor/cytotoxic cells are almost completely restricted to the cords of Billroth in the red pulp. The cells in the mantle zone of germinal centres and in the primary follicles in lymph nodes, tonsils and spleens have B-cell properties (BA1-, HLA-DR-, and CR1-positive). The cells in the germinal centres show a similar staining pattern (HLA-DR-, and partly CR1-positive). Follicles and T-cell-dependent areas have specific dendritic cells, each with a specific staining pattern: the dendritic reticulum cell (DRC) of the follicle stain with CR1, HLA-DR, BA2 and alpha HM1; the interdigitating cell of the T-cell areas in the lymph node, tonsil and spleen stain with HLA-DR and BA1. Moreover, large dendritic OKT6-positive cells are found in the T-cell areas of some of the peripheral lymph nodes, and are probably Langerhans cells. It is concluded that human lymph nodes and tonsils have an identical compartimentalisation, clearly differing from the spleen in cellular organization.     

Value of lymphocyte marker studies in diagnostic cytopathology

The separation of lymphoreticular neoplasms from poorly differentiated epithelial and mesenchymal tumors and reactive processes can be a difficult problem in cytopathology. Immunodiagnostic techniques can be applied to cytologic specimens to detect cellular antigens, which may aid in their proper identification. We have reviewed 67 cytologic specimens in which immunoperoxidase techniques were employed using antibodies to common leukocyte antigen (HLE1), B-cell markers (B1, Leu 12 and kappa and lambda light chains), T-cell markers (Leu 1, OKT11, Leu 12 and kappa and lambda light chains), T-cell markers (Leu 1, OKT11, OKT4 and OKT8) and monocytes (OKM1 and LeuM1). These specimens included 33 body cavity fluids (21 pleural, 8 ascitic and 4 pericardial), 22 cerebrospinal fluids (CSF) and 12 fine needle aspirates (4 brain, 1 adrenal, 2 liver, 1 kidney, 3 retroperitoneal masses and 1 lymph node). The marker studies confirmed the initial cytomorphologic diagnoses in 31 specimens and modified the final diagnoses in 16 specimens. Markers in 20 specimens were noncontributory due to low cellularity or technical difficulties. Two problems may limit the usefulness of these procedures. First, many of the CSF specimens contained too few cells for adequate processing. Second, the mesothelial cells from pleural specimens often stained with HLE1. Our findings indicate that marker studies are of value in the diagnosis of problematic cases presenting as undifferentiated tumors in cytopathology.     

Ultrastructural analysis of HNK-1+ cells in human peripheral blood and lymph nodes

HNK-1 positive (HNK-1+) cells in human peripheral blood and lymph nodes were comparatively analysed by means of immunohistochemistry and immunoelectron microscopy. In peripheral blood, the HNK-1+ cells were grouped into large granular lymphocytes (LGLs), small lymphocytes and intermediate forms, all of which had many fine cytoplasmic processes. Except for smooth-surfaced lymphocytes, they could not be distinguished from helper/inducer T (OKT4/Leu3a) cells and suppressor/cytotoxic T (OKT8/Leu2a) cells. In double staining, HNK-1+T3- cells and HNK-1+T3+ cells could not be clearly distinguished in terms of morphology, although the former contained many LGLs. The HNK-1+ cells in the lymph nodes accumulated in the light zones of the germinal centers (GCs). These cells were small to medium-sized lymphocytes with few electron-dense granules and exclusively co-expressed helper/inducer T cell antigens (HNK-1+T4+). Their cytoplasmic projections were interwoven with those of the follicular dendritic cells which trap immune complexes for a long duration. These configurations suggest that HNK-1+T4+ cells in GCs are engaged in an immunological regulation of germinal center cells. On the other hand, large blastic HNK-1+ cells were scattered outside the GCs and some of them were in the process of mitosis. Furthermore, HNK-1+LGL-like cells with a few large electron-dense granules were rarely seen. These observations indicate that the HNK-1+ cells in the lymph nodes may proliferate outside GCs and differentiate into LGLs with a strong natural killer function.     

Cell subpopulations in the paracortical area of the normal human lymphnode defined by monoclonal antibodies

The cell subpopulations of normal human lymphnode were studied by immunohistochemical and immunoelectronmicroscopy techniques carried out by using monoclonal antibodies. OKT3 positive cells were demonstrated overwhelming in comparison to both OKT4 positive cells and OKT8 positive cells; furthermore la positive cells, dendritic in shape, were observed. Our results, compared to literature data, confirm that paracortex of limphnode is a T-dependent area. Moreover a possible role of the la positive dendritic cells in the immunological education of T helper-inducer lymphocytes was hypotised. Finally, our immunoelectronmicroscopic findings prove that OKT4 positive cells and OKT8 positive cells show different ultrastructural patterns.     

HIV envelope proteins are bound by human epidermal Langerhans cells by a binding site which differs from the site on the CD4 molecule, and are internalized by receptor endocytosis

Langerhans cells (LC) are epidermal dendritic cells which express several surface antigens among them the CD4 antigens. We investigated the fate of HIV envelope glycoproteins (gp 120 and gp 160) incubated with healthy human trypsinized LC in suspension. After trypsin treatment only the epitope for OKT4 appeared to be resistant. In absence of antigenic sites identified by OKT4A, Leu 3a or BL4, LC fixed and internalized gp 120 or gp 160 recombinant HIV proteins. This finding support the hypothesis that there exists at the surface of LC a second molecule which may act as a HIV receptor.     

Detection of distinct subpopulations of Langerhans cells by flow cytometry and sorting

Flow cytometry was found to be a very appropriate tool for the study of Langerhans cells (LC), which represent a minor cell population (2-3%) of human epidermis, and allowed us to obtain new phenotypic, functional, and cell cycle data on these rare cells. The phenotypic analysis of cell surface antigens demonstrates the existence of two subpopulations of LC: the former is HLA-DR+ and OKT 6+ (about 90% of total HLA-DR+ cells) and the latter is HLA-DR+ and OKT 6- (about 10% of total HLA-DR+ cells). These subpopulations of LC are both able to stimulate the proliferation of peripheral blood lymphocytes (PBL) in the presence of keratinocytes i.e., in mixed skin lymphocyte reaction (MSLR). Analysis of the cell cycle could be performed on OKT 6+ LC. Results show that they can be found in the various phases of the cell cycle, suggesting that the large majority of Langerhans cells are able to proliferate in situ in normal human epidermis.     

Induction of monocyte procoagulant activity with OKT3 antibody

OKT3 monoclonal antibody (MoAb), a mouse MoAb against cluster of differentiation 3 (CD3) molecule, induced a large amount of procoagulant activity (PCA) in human peripheral blood mononuclear cells (PBM). The PCA-inducing capability in OKT3 MoAb was abolished by absorption with T lymphocytes or Sepharose-conjugated antibody to mouse IgG. Most of the PCA in PBM was associated with monocytes. There was a dose-dependent increase in PCA when increasing numbers of T cells were added to the monocytes in the presence of OKT3 MoAb. OKT3 MoAb did not induce PCA in either T cells or monocytes alone. T cells pulsed with OKT3 MoAb only in the presence of monocytes could induce PCA in monocytes. Culture supernatants (CS) from PBM stimulated with OKT3 MoAb did not enhance PCA in monocytes; however, it did induce PCA in the human monocyte-like cell line (U937) which differs in some properties from monocytes; this activity could be abolished by the MoAb against human interferon-gamma (IFN-gamma). Nevertheless, neither human IFN-gamma nor interleukin 1 or 2 had significant direct effect in inducing PCA in U937 cells; CS from either monocytes or T cells alone stimulated with OKT3 MoAb did not induce PCA in U937 cells. This apparent discrepancy suggests that there may be factors in CS that induce PCA in U937 cells only in the presence of IFN-gamma. The PCA induced in monocytes or U937 cells was tissue factor-like because of the dependence on coagulation factors V, VII, and X. These observations suggest that OKT3 MoAb is a potent T cell-dependent monocyte PCA inducer and stimulates T cells only in the presence of monocytes. The direct cellular interaction between monocytes and stimulated T cells appears to be necessary to elicit monocyte PCA with OKT3 MoAb stimulation. Thus, monocytes may play a dual role, not only as effector cells, but also as cells that collaborate with T cells after OKT3 MoAb stimulation so as to produce PCA.     

In vitro binding and internalization of HIV envelope glycoproteins by human epidermal langerhans cells does not require the CD4-GP120-binding site

Langerhans cells (LC) are epidermal dendritic cells which express several surface antigens, among them the CD4 antigens. Recent data demonstrated that LC constitute target and storage cells for HIV. To better understand the interactions between HIV and LC, we investigated, in the present work, the fate of HIV envelope glycoproteins (gp120 and gp160) incubated with healthy human trypsinized LC in suspensions.After trypsin treatment, only the epitope for OKT4 appeared to be resistant on LC. In the absence of antigenic sites identified by OKT4A, Leu3a or BL4 (epitopes implicated in HIV binding), LC bound and internalized recombinant HIV gp120 or gp160.This finding supports the hypothesis that there exists at the surface of LC a second molecule which may act as an HIV receptor.     

Regulation of influenza virus-specific cytotoxic T cell responses by monoclonal antibody to a human T cell differentiation antigen

The results in this report indicate that the OKT3 monoclonal antibody, which is specific for a human T cell differentiation antigen present on 90 to 95% of peripheral T cells, can exert several effects that regulate the generation and expression of human influenza virus-immune cytotoxic T lymphocytes (CTL). The OKT3 antibody, but not OKT1 or OKT11 (which bind to all peripheral T cells), is able to inhibit anti-influenza CTL effector cell activity. An F(ab')2 preparation of OKT3 IgG were as effective as whole IgG for the inhibition of CTL effectors, indicating that the inhibitory activity of the antibody was not a function of the Fc portion of the molecule. OKT3 IgG and OKT3 F(ab')2 fragments (but not OKT4, OKT8, or OKI were able to inhibit the generation of anti-influenza CTL. The culture of human lymphoid cells with OKT3 in the presence or absence of influenza virus induced radioresistant cells that could suppress the CTL response of fresh autologous lymphocytes to influenza. These results suggest that T cell functions can be regulated by signals that are initiated by the binding of antibody to cell surface molecules that may not be related to the T cell antigen-specific receptor(s).     

Regulation of cellular immune response against autologous human melanoma. I. Evidence for cell-mediated suppression of in vitro cytotoxic immune response

The potential existence of down-regulation of cytotoxic immune response against an autologous human melanoma line was investigated as a possible explanation for cytotoxic unresponsiveness against the autologous melanoma cells. The melanoma cell line, PJ-M, was established and lymph node resident lymphocytes (LNL) were isolated from a lymph node which was partially infiltrated with the melanoma cells. Autologous peripheral blood lymphocytes (PBL) were sensitized in in vitro co-culture (IVC) against radiated PJ-M cells in the presence or absence of PJ-M-sensitized LNL and enriched suppressor (OKT8+) or inducer (OKT4+) LNL populations, and were assayed for cytotoxicity in a 4-hr 51Cr-release microcytotoxicity assay. Significant cytotoxic response against PJ-M could be generated in the PBL, but not in the LNL. The addition of sensitized, unfractionated LNL, OKT8+, or OKT4+ LNL populations abrogated cytotoxic response in the PBL against PJ-M. The suppression of cytotoxic response was induced selectively against the PJ-M targets, because IVC of PBL in the presence of the sensitized LNL did not affect the generation of polyclonal cytotoxic alloreactivities, nor did they abrogate the generation of cytotoxic response against allogeneic targets in IVC against the corresponding allogeneic targets. These results suggest the possibility that cytotoxic immune response against the autologous melanoma cells might have been suppressed by the individual's own immunoregulatory circuit.     

History of immunology. Development of the concept of immunologic specificity, I

The induction of differentiation in human malignant T-lymphoblastic cell lines MOLT-3 and Jurkat by the tumor promoter, 12-O-tetradecanoylphorbol 13-acetate (TPA) was examined using the monoclonal antibodies OKT3, OKT4, OKT6, and OKT8 which are known to react with human T-cell differentiation antigens. It was found that in the presence of nanomolar concentrations of TPA the proportion of OKT3⁺ (mature T-cell marker) cells increased while the proportion of OKT4⁺, OKT6⁺, and OKT8⁺ (relatively immature T-cell markers) cells decreased. These changes in the distribution of the OKT antigens in MOLT-3 cells were found to be more prominent with MOLT-3 cells than when the Jurkat cells were used. In studies using a double labeling approach it was found that although the OKT3⁺ and E-rosette-positive (E⁺) cells appeared to belong to the same subpopulations of MOLT-3 cells, the OKT3 antigen was probably not related to the receptor for sheep erythrocytes because adsorption of the OKT3 antibody did not block E-rosette formation. Studies using the DNA synthesis inhibitor, arabinosylcytidine (ara-C) also indicate that DNA synthesis was not required for the induction of more mature T-cell antigens in the malignant T-cell lines by TPA. These studies, taken together with our earlier reports, support the conclusion that namomolar concentrations of TPA can induce differentiation in these malignant T-cell lines. Furthermore we have shown that the T-cell hybridoma antibodies are useful markers to detect differentiation changes in human T cells.     

Phenotypic Analysis of New World Primate Mononuclear Cell Surface Antigens

We have applied a panel of monoclonal antibodies against antigens present on the surface of human mononuclear cells to the study of mononuclear cell surface antigens expressed by seven species of New World primates. Antibodies to the sheep erythrocyte receptor (OKT11a) to a thymocyte antigen (OKT10), to the I region of the major histocompatibility locus (OKIa), and to an antigen found on the surface of human monocytes (OKM1) cross-reacted with mononuclear cell surface antigens of most of the species studied. Antibodies to antigens which have been correlated with functional capabilities in the human system (OKT4, OKT5, OKT8, 3A1) were much less reactive with platyrrhine mononuclear cells. These reagents may be quite useful in studies of primate phylogeny and immunology.     

Varicella-zoster virus infection of human dendritic cells and transmission to T cells: implications for virus dissemination in the host

During primary varicella-zoster virus (VZV) infection, it is presumed that virus is transmitted from mucosal sites to regional lymph nodes, where T cells become infected. The cell type responsible for VZV transport from the mucosa to the lymph nodes has not been defined. In this study, we assessed the susceptibility of human monocyte-derived dendritic cells to infection with VZV. Dendritic cells were inoculated with the VZV strain Schenke and assessed by flow cytometry for VZV and dendritic cell (CD1a) antigen expression. In five replicate experiments, 34.4% +/- 6.6% (mean +/- SEM) of CD1a(+) cells were also VZV antigen positive. Dendritic cells were also shown to be susceptible to VZV infection by the detection of immediate-early (IE62), early (ORF29), and late (gC) gene products in CD1a(+) dendritic cells. Infectious virus was recovered from infected dendritic cells, and cell-to-cell contact was required for transmission of virus to permissive fibroblasts. VZV-infected dendritic cells showed no significant decrease in cell viability or evidence of apoptosis and did not exhibit altered cell surface levels of major histocompatibility complex (MHC) class I, MHC class II, CD86, CD40, or CD1a. Significantly, when autologous T lymphocytes were incubated with VZV-infected dendritic cells, VZV antigens were readily detected in CD3(+) T lymphocytes and infectious virus was recovered from these cells. These data provide the first evidence that dendritic cells are permissive to VZV and that dendritic cell infection can lead to transmission of virus to T lymphocytes. These findings have implications for our understanding of how virus may be disseminated during primary VZV infection.     

Immature T lymphocytes in human cord blood identified by monoclonal antibodies: a model for the study of the differentiation pathway of T cells in humans

The reactivity of human cord blood lymphocytes was assessed against a panel of monoclonal antibodies (MoAb). The mean proportion of OKT3+ cells (pan-T) was significantly lower in cord blood (52 +/- 13.8%; mean +/- SD) compared with that of adult blood (75 +/- 8.9%) and paralleled well with the E-rosette-forming capacity (50 +/- 16.3%). Both the proportions of OKT4+ cells (helper/inducer phenotype) and of OKT8+ cells (suppressor/cytotoxic phenotype) were significantly reduced in cord blood (43 +/- 11.8% vs 50.3 +/- 7.4% and 20 +/- 10.3% vs 25.6 +/- 6.0%, respectively), while the overall OKT4/OKT8 ratio was increased compared with adult blood (2.87 +/- 1.83 vs 2.04 +/- 0.61). Unlike adult blood, in 30 of the 35 samples of cord blood an overlap was observed between the total proportion of OKT4+ and OKT8+ cells (65 +/- 15.2%) and that of OKT3+ cells (52 +/- 14.3%). Although small numbers of cells coexpressing both antigens were occasionally found, double-staining analysis showed that the overlap in cord blood was mostly due to an expanded proportion of OKT3 (Leu-4)-/OKT8 (Leu-2)+ cells. Relevant proportions of OKT6+ (common thymocyte antigen) and OKT10+ (thymocytes, activated T cells, precursor cells) cells were found in cord blood as opposed to adult blood (10.8 +/- 8.6% vs 0.6 +/- 0.6% and 67 +/- 18.0% vs 8 +/- 2.1%, respectively), while terminal deoxynucleotidyl transferase-positive cells were observed only in two samples of cord blood. A small proportion of T cells (E-rosette+) reacted with the MoAb OKIa1 (HLA-DR). Finally, the proportion of cord blood cells recognized by the MoAb Leu-7 (HNK-1 clone) was almost negligible compared with adult blood (2.8 +/- 2.4% vs 15 +/- 7.5%). These data confirm the immaturity and heterogeneity of cord blood lymphocytes and demonstrate the presence at birth of circulating lymphocytes which express a surface phenotype reminiscent of that found in the late stages of intrathymic differentiation and in some human T-cell leukemias. Human cord blood may thus represent a suitable model for the study of the differentiation pathway of normal and pathological T-cells in humans.     

Differentiation of Langerhans Cells from Interdigitating Cells Using CD1a and S-100 Protein Antibodies

The present study shows that Langerhans cells can be differentiated from Interdigitating cells at the light microscopic level. Superficial lymph nodes and skin taken from necropsies and the lymph nodes of dermatopathic lymphadenopathy (DPL) were used for this experiment. Sections of lymph node and skin were embedded using the acetone, methyl benzoate and xylene (AMeX) method and dendritic cells were immunostained with anti S-100 protein antibody (S-100, and OKT-6 (CD1a) using the restaining method. Langerhans cells in the skin were positive for both CD1a and S-100. Dendritic cells positive for both CD1a and S-100, and dendritic cells positive for S-100, but not for CD1a were observed in superficial lymph nodes. In normal superficial lymph nodes, there were more interdigitating cells than Langerhans cells. The majority of the dendritic cells in the DPL were Langerhans cells. We conclude that the S-100 and CD1a positive cells are Langerhans cells, and the S-100 positive-CD1a negative cells are interdigitating cells.     

Thymosin beta 4 induced phenotypic changes in Molt-4 leukemic cell line

Thymosin beta 4 was tested for its ability to induce phenotypic changes in the human T-cell line Molt-4. Cells were cultured with nanogram concentrations of thymosin beta 4 for up to 16 days and were analyzed with a panel of monoclonal antibodies, sheep erythrocyte rosetting, peanut agglutinin binding (PNA) and an antibody to the enzyme, terminal deoxynucleotidyl transferase (TdT). Thymosin beta 4 induced Molt-4 cells to reduce the expression of a T-cell lineage specific antigen, with preferential expression on T blast-cells, detected by WT 1 monoclonal antibody. Thymosin beta 4 also induced an increase in sheep erythrocyte rosettes and PNA binding as well as an increased expression of OKT 11 A and OKT 8 in Molt-4 cells. TdT was found to be unchanged, however. Analysis of thymosin beta 4-treated cells with other monoclonal antibodies (OKT 3, OKT 6, OKT 9) showed no change when compared to controls. These results showed that thymosin beta 4 is capable of inducing phenotypic changes in Molt-4 cells. Such changes may represent a differentiation process of these cells through the early stages of the maturation process of thymus-dependent lymphocytes, albeit not to the stage of mature T cells.     

Differentiation of Langerhans Cells from Interdigitating Cells Using CD1a and S-100 Protein Antibodies

The present study shows that Langerhans cells can be differentiated from Interdigitating cells at the light microscopic level. Superficial lymph nodes and skin taken from necropsies and the lymph nodes of dermatopathic lymphadenopathy (DPL) were used for this experiment. Sections of lymph node and skin were embedded using the acetone, methyl benzoate and xylene (AMeX) method and dendritic cells were immunostained with anti S-100 protein antibody (S-100, and OKT-6 (CD1a) using the restaining method. Langerhans cells in the skin were positive for both CD1a and S-100. Dendritic cells positive for both CD1a and S-100, and dendritic cells positive for S-100, but not for CD1a were observed in superficial lymph nodes. In normal superficial lymph nodes, there were more interdigitating cells than Langerhans cells. The majority of the dendritic cells in the DPL were Langerhans cells. We conclude that the S-100 and CD1a positive cells are Langerhans cells, and the S-100 positive-CD1a negative cells are interdigitating cells.     

Subsets of human large granular lymphocytes (LGL) exhibit accessory cell functions

The present study shows that human large granular lymphocytes (LGL) depleted of OKT3 (T lymphocytes) and Leu-M1-positive (monocytes) cells exhibit accessory cell function for the T lymphoproliferative responses to the soluble stimulants Staphylococcus protein A (SpA) or Streptolysin O (SLO), as well as to surface antigens in the autologous and allogeneic mixed leukocyte reaction (MLR). Fractionation of LGL into subsets according to their reactivity with alpha OKT11, alpha DR, and alpha OKM1 MoAb led to the identification of the subset(s) of LGL with OKT11+, DR+, OKM1+ phenotype as the antigen-presenting cell (APC), whereas the DR-, OKM1- subset(s) of LGL was completely ineffective. Furthermore, virtually all the natural killer (NK) activity of LGL was associated with OKT11+ and OKM1+, DR+ LGL that exerted the observed APC function, suggesting that NK-active cells may also act as effective APC for T lymphocyte activation. These results indicate that human LGL with NK activity may exert other noncytotoxic functions and may play a major role in immunoregulation.