A monoclonal antibody against altered LFA-1 induces proliferation and lymphokine release of cloned T cells

Molecular markers (PECAM-1, ICAM-3, HLA-DR) determine prognosis in primary non-Hodgkin's gastric lymphoma patients

AIM

To investigate the prognostic significance of PECAM-1, ICAM-3 and HLA-DR antigens in patients with primary non-Hodgkin's gastric lymphoma.

METHODS

We immunohistochemically studied PECAM-1, ICAM-3 and HLA-DR antigen expression in 36 B-cell MALT-type primary gastric lymphoma patients. Ten non-malignant and ten healthy gastric tissue specimens were used as controls. Clinicopathological and survival data were correlated with the staining results.

RESULTS

HLA-DR antigen expression was detected in 33 gastric lymphoma patients (91.7%) and 6 non-malignant patients (54.5%). PECAM-1 stained tumor cells of 10 patients (27.8%), endothelial cells of 9 patients (25%) and inflammatory infiltrate of 4 patients (40%) with benign gastric disease. ICAM-3 expression was observed on the tumor cells of 17 patients (47.2%), while 5 non-malignant patients (50%) were stained positive as well. None of the healthy controls was stained for any of the genes studied. In the multivariate analysis, HLA-DR antigen and PECAM-1 were proved to be statistically significant independent prognostic factors associated with a favourable and an unfavourable prognosis respectively (P=0.009 and P=0.003). In the univariate analysis, PECAM-1(+)/ICAM-3(-) and HLA-DR(-)/ICAM-3(-) patients exhibited a significantly decreased overall survival compared to those with the exactly opposite gene expression patterns (P=0.0041 and P=0.0091, respectively). Those patients who were HLA-DR(+)/ICAM-3(+)/PECAM-1(-) (n=8) had a significantly higher survival rate compared to the rest of the group (n=24) (P=0.0289).

CONCLUSION

PECAM-1, ICAM-3 and HLA-DR are representative markers of tumor expansion potential and host immune surveillance respectively. Their combined use may help us to identify high-risk patients who could benefit from more aggressive therapeutic protocols.

PMA-treated K-562 leukemia cells mediate a TH2-specific expansion of CD4+ T cells in vitro

Highly enriched preparations of human CD3+CD4+ T-lymphocytes were stimulated with mitogen or OKT3 to determine the capacity of K-562 cells to function as accessory cells. Phorbol 12-myristate 13-acetate (PMA)-treated K-562 cells were induced to differentiate along the megakaryocytic lineage and could supplant monocyte-accessory cell function. Intracytoplasmic analysis of interleukin-4 (IL-4) and interferon-gamma (IFN-gamma) established that IL-4, and not IFN-gamma, was preferentially produced by the activated lymphocytes. This polarized stimulation is compatible with a type 2 or humoral immune response of purified T cells co-cultured with differentiated K-562 cells in vitro, and may have implications in immunoregulation due to disease progression.

Co-stimulation of T cells results in distinct IL-10 and TNF-alpha cytokine profiles dependent on binding to ICAM-1, ICAM-2 or ICAM-3

The LFA-1 adhesion molecule is involved in cell adhesion events of leukocytes through binding to ICAM-1, ICAM-2 and ICAM-3. Whether binding to either of these ligands similarly affects co-stimulation of T cells and cytokine secretion is unknown. We demonstrated that LFA-1 co-stimulation under suboptimal concentrations of anti-CD3 monoclonal antibodies resulted in high, intermediate and weak proliferation of T cells on ICAM-1, -2, and -3, respectively, which correlates with the distinct affinities of LFA-1 for these ligands. Furthermore, we investigated whether binding to ICAM-1, -2 or -3 induced different cytokine profiles, thus regulating T helper cell function. Granulocyte-macrophage colony-stimulating factor and IFN-gamma were secreted in high amounts, whereas IL-2, IL-4 and IL-5 could not be detected. Interestingly, we observed that LFA-1/ICAM-1 co-stimulation of T cells resulted in high production of the Th2 cytokine IL-10 compared to ICAM-2 or ICAM-3 co-stimulation. In contrast, ICAM-2 and ICAM-3 induced a much stronger secretion of the Th1 cytokine TNF-alpha compared to LFA-1/ICAM-1 induced co-stimulation, despite the lower proliferation rate. These results demonstrate that besides facilitating cell adhesion, LFA-1 serves as a potent co-stimulatory molecule by inducing different cytokine patterns depending on the ligand bound.

The equine homologue of LFA-1 (CD11a/CD18): cellular distribution and differential determinants

The equine homologue of the leucocyte integrin LFA-1 (CD11a/CD18) has been characterized using a panel of four monoclonal antibodies (mAbs). The antibodies labelled almost all leukocytes, thymocytes and lymph node cells from normal horses, and immunoprecipitated two noncovalently associated polypeptides with molecular weights of 180 kDa and 100 kDa, respectively. The antigen recognized by one mAb could be precipitated by another in this cluster in a sequential immunoprecipitation assay. The mAbs, however, did not block the activities on lymphocyte function of one another. A mAb to the beta subunit of human LFA-1 cross-reacted with equine LFA-1, but an antibody to its alpha subunit did not, suggesting that the beta subunit of the leukocyte integrin may be more highly-conserved. Functionally, H20A and a human CD18 antibody (MHM23) inhibited phorbol ester-mediated homotypic lymphocyte aggregation, whereas mAb CZ3.2 induced rather than inhibited the homotypic cell aggregation. The formation of lymphocyte aggregates induced by CZ3.2 was not blocked by the inhibitory antibodies H20A or MHM23. CZ3.1 seemed to have little inducible or inhibitory effects on homotypic cell aggregation. The mAb CZ3.1 defined a unique LFA-1 determinant present on granulocytes, but absent on lymphocytes in members of an extended horse family, in contrast to the other antibodies which labelled both granulocytes and lymphocytes from these animals. In all other horses tested, no differences in reactivity of CZ3.1 and the other LFA-1 antibodies were observed when the antibodies were tested on lymphocytes or granulocytes. Our results indicate that common epitopes are shared' between human and equine LFA-1, and that the described panel of monoclonal antibodies identifies distinct determinants present on the equine LFA-1 molecule. The following monoclonal antibodies used in this study were given official workshop designations at the Second International Workshop on Equine Leukocyte Antigens (Lunn et al., 1998) CZ3.1 (Cor) = W45; CZ3.2 (Cor) = W77.

Effects of cytokines combined with high-dose gamma irradiation on the expression of major histocompatibility complex molecules and intercellular adhesion molecule-1 in human ovarian cancers

Tumor cells from 7 freshly isolated human ovarian tumors and 2 continuous human ovarian cancer cell lines were analyzed for their surface expression of MHC class-1, class 11 and ICAM-1 surface antigens before and after exposure to gamma-irradiation and/or the cytokines TNF-alpha plus IFN-gamma. All 7 fresh tumors expressed high levels of MHC class 1 and 1CAM-1 antigens, and levels were markedly up-regulated after exposure to TNF-alpha plus IFN-gamma Similarly, class-11 antigens were either induced (3 out of 7 tumors) or significantly up-regulated by TNF-alpha plus IFN-gamma. Exposure to high doses of gamma-irradiation also increased the expression of MHC class-1 and ICAM-1 antigens, albeit to a modest degree. MHC class 1 and ICAM-1 antigens expression was much lower on continuous human ovarian cell lines than on the fresh tumors. Exposure of these cells to TNF-alpha plus IFN-gamma markedly up-regulated antigen expression to levels comparable to those expressed on the freshly isolated tumors. With the established ovarian cell lines, removal of cytokines caused a rapid down-regulation of antigen expression to basal levels within 6 days, while in the fresh tumors a low level of up-regulation was still present at this time. In contrast, exposure to cytokines followed by high-dose gamma-irradiation resulted in a highly significant and long-lasting expression of each surface antigen which was either up-regulated or induced by the cytokines. These data indicated that the combination of these modalities may be beneficial in generating optimal antigen expression for use of tumor cells in vaccine studies.

Antibodies that activate beta 2 integrins can generate different ligand binding states

A human erythroleukemic cell line (K562) that does not normally express beta 2 integrins has been transfected with the genes encoding these integrins. The resulting cell lines show minimal background adhesion but can be stimulated to bind to appropriate substrates when activated with either of two different antibodies to CD18. The two antibodies appear to generate different ligand binding states in LFA-1 such that different members of the ICAM family are recognized. Antibody-activated complement receptor type 3 and p150,95-transfected cells bind protein-coated surfaces, although they require slightly different activation conditions for optimal binding.

Expression of cell interaction molecules by immature rat thymocytes during passage through the CD4+8+ compartment: developmental regulation and induction by T cell receptor engagement of CD2, CD5, CD28, CD11a, CD44 and CD53

Rat thymocytes of the T cell receptorlow (TcRlow) CD4+8+ subset which is the target of repertoire selection are heterogeneous with respect to expression of the cell interaction (CI) molecules CD2, CD5, CD11a/CD18 (LFA-1), CD28 and CD44. We show that this heterogeneity is due to the developmental regulation of these CI molecules during passage through the CD4+8+ compartment, and to up-regulation by TcR engagement. Thus, cohorts of CD4+8+ cells differentiating synchronously in vitro from their direct precursors, the immature CD4-8+ cells, were homogeneous with regard to CI molecule expression. Upon entry into the CD4+8+ compartment, they expressed relatively high levels of CD2 and CD44, and moderate levels of CD5, CD28 and CD11a. CD2, CD28 and CD44 were slightly down-regulated during the following 2 days, whereas CD5 slightly increased and CD11a remained constant. TcR stimulation using immobilized monoclonal antibodies resulted in rapid and dramatic up-regulation of CD2, CD5 and CD28 and, to a lesser extent, of CD11a and CD44. Finally CD53, a triggering structure absent from unstimulated CD4+8+ thymocytes was also rapidly induced by TcR stimulation. Inclusion of interleukin (IL)-2, IL-4, or IL-7 in this in vitro differentiation system did not affect the levels of CI molecules studied. Since the high levels of CI molecules induced by TcR-stimulation correspond to those found in vivo on TcRintermediate thymocytes known to be undergoing repertoire selection, these results suggest that upregulation of CI molecules by TcR engagement provides a mechanism by which thymocytes that have entered the selection process gain preferential access to further interactions with stromal and lymphoid cells in the thymus.

Adhesion receptors in lymphocyte activation

The past several years have seen significant progress in understanding the role of T lymphocyte coreceptors in adhesion and activation. New insights have been gained in several areas: the avidity regulation of beta 1 and beta 2 integrins and their role in signal transduction; the regulation of CD8 avidity; the role of Lck in CD4 coreceptor activity; and the novel role for CD2 adhesion in the T cell antigen response.

Interaction between neutrophils and endothelium

The endothelial permeability associated with cardiopulmonary bypass in children is due, at least in part, to an inflammatory response. Neutrophils and their relationship with endothelium are fundamental to the production of capillary leak. This review discusses current concepts of the mechanisms involved in adhesion between neutrophils and the endothelium and its control. Evidence for modulation of these changes during bypass in children is reviewed. We have shown that cardiopulmonary bypass in children is associated with down-regulation (or shedding) of L-selectin and up-regulation of expression of CD11b/CD18. There may be a role for the cytokine interleukin-8 in the modulation of this process. We have demonstrated in vitro that certain of the procedures associated with bypass are associated with up-regulation of circulating neutrophil adhesion molecules and with the release of interleukin-8. These factors include changes in temperature and circulation. More research is required to tease out the most important components of the mechanisms of adhesion, and clinical correlations must be defined.

Signaling through the LFA-1 leucocyte integrin actively regulates intercellular adhesion and tumor necrosis factor-alpha production in natural killer cells

The LFA-1 leucocyte integrin is known to participate in natural killer (NK) cytolytic activity, mediating effector target interactions. The possibility that LFA-1 may also play an active regulatory role in NK cells has been explored. To this end, we have employed a monoclonal antibody (HP1N) raised against recombinant interleukin-2 (rIL-2)-activated NK cells, which recognizes the alpha chain of the LFA-1 heterodimer (CD11a). In contrast to other anti-CD11a mAb the HP1N and its F(ab')2 fragment did not affect NK cell-mediated cytotoxicity and triggered a strong homotypic adhesion of NK cells and other LFA-1+ cells. Cellular aggregation was inhibited by anti-CD18 mAb, anti-ICAM-1 mAb, and other anti-CD11a mAb. Remarkably, the HP1N mAb was also shown to induce tumor necrosis factor-alpha (TNF-alpha) production from NK cells upon costimulation with anti-CD16 mAb. Such an effect appeared to be independent from homotypic adhesion since it took place in Mg(2+)-free medium, where NK cell aggregation was inhibited. Moreover, incubation with the HP1N mAb triggered a Ca2+ influx into the cytosol; this effect was clearly observed upon cross-linking of cell bound HP1N and was also substantiated with other anti LFA-1 (CD11a and CD18) mAb. Taken together these results indicate that the LFA-1 molecule is capable of transducing signals in NK cells, which regulate the intercellular interaction with its ligand, and enhance the activation via Fc gamma receptor type III.

Expression of intercellular adhesion molecule 1 (ICAM-1) during the development of invasion and/or metastasis of gastric carcinoma

In this study, using two-color flow-cytometric analysis, we examined the expression of histocompatibility locus antigens (HLA) classes I and II, and intercellular adhesion molecule 1 (ICAM-1) in 10 cases of normal gastric mucosa, 13 cases of primary carcinoma on the stomach, 16 cases of metastatic carcinoma from malignant ascites in patients with gastric carcinoma and 14 samples of their cultured carcinoma cells. Compared with normal gastric mucosa, HLA class I were highly expressed in a considerable number of tumor cells in each experimental group. The expression of HLA class II tended to reduce in the order of normal gastric mucosa, primary gastric carcinoma and peritoneal-effusion-associated carcinoma. Altogether, 85.7% of cases of cultured tumor cells showed abrogation and loss of HLA class II. The ICAM-1 molecule was not detected on normal gastric epithelial cells. In few cases, carcinoma cells from large volumes of tumor located in the stomach showed detectable amounts of ICAM-1. On the other hand, all of the metastatic carcinoma cells from peritoneal effusions showed a high level of expression of the ICAM-1 molecule. The expression of ICAM-1 on adenocarcinoma cells was maintained and/or augmented by in vitro cultivation with tumor-infiltrating lymphocytes (TIL). Furthermore, two-color fluorescence-activated cell sorting analysis of TIL revealed that significant correlation was observed between the expression of ICAM-1 and the degree of TIL, composed mainly of CD3+ T cells including CD8+CD11b-, CD8+CD28+, CD8+S6F1+ and CD4+Leu8+, and CD57+CD16- and CD57+CD16+ NK cells, and HLA-DR+LeuM3+ macrophages.

Monoclonal antibodies targeting murine LFA-1 induce LFA-1/ ICAM-1-independent homotypic lymphocyte aggregation

We have identified three anti-murine LFA-1 alpha monoclonal antibodies (M17/4.2, G-48, and FD441.8) which are capable of inducing homotypic aggregation of murine T cell lines (3A9 EL-4 cells). The LFA-1-induced aggregation is temperature-dependent, necessitates metabolic energy, and requires an intact cytoskeleton, but is independent of transcription and protein synthesis. The aggregation is inhibited in Ca2+ and Mg2+ free media and is also blocked with EDTA and EGTA. The aggregation does not involve protein kinase A or C or changes in intracellular calcium. The LFA-1 alpha-induced homotypic aggregation is inhibited with LFA-1 beta antibodies, but not with antibodies targeting ICAM-1, VCAM-1, VLA-4, or CD2. 3A9 cells do not express the LFA-1 ligand ICAM-1, whereas EL-4 cells express moderate amounts of ICAM-1. Thus, targeting LFA-1 alpha with mAb results in homotypic aggregation of T cell lines which is independent of ICAM-1/LFA-1 interactions, but may involve other LFA-1 ligands such as ICAM-2 or ICAM-3. Alternatively, LFA-1 may function as a signaling molecule, triggering other yet to be defined adhesion molecules to interact.

Induction of homotypic T cell adhesion by triggering of leukocyte function-associated antigen-1α (CD11a): differential effects on resting and activated T cells

The leukocyte integrin LFA-1 (CD11a/CD18) plays a key role in many adhesive interactions involving cells of the immune system. Recently, it has been shown that LFA-1 is not only involved in cell adhesion, but that stimulation of LFA-1 can also contribute to cell activation. We now demonstrate that triggering of LFA-1 on T lymphocytes by monoclonal antibodies (mAb) against the LFA-1 alpha chain, but not against the LFA-1 beta chain, promotes cell adhesion. Induction of homotypic adhesion was only observed in T cells that had been pre-activated with anti-CD3 and not in resting peripheral blood T lymphocytes. The induced homotypic adhesion is mediated by LFA-itself, because it was inhibited by anti-LFA-1 beta mAb. This notion is supported by the temperature and divalent cation dependence which is characteristic of LFA-1-mediated adhesion. mAb against ICAM-1 (CD54) did not block LFA-1 alpha-induced adhesion. The sensitivity of LFA-1 alpha-induced adhesion to H7, which prevents the activation of protein kinase C and protein kinase A, and to cytochalasin B, which inhibits microfilament formation, suggests that the activation of the LFA-1 pathway through the LFA-1 alpha chain involves cell activation and requires an intact cytoskeleton.

Distinct cellular functions mediated by different VLA integrin alpha subunit cytoplasmic domains

To characterize VLA alpha subunit cytoplasmic domain functions, unaltered alpha 2 cDNA (called X2C2) and two chimeric cDNAs (called X2C5 and X2C4) were constructed with extracellular alpha 2 domains and cytoplasmic alpha 2, alpha 5, and alpha 4 domains respectively. Upon transfection into rhabdomyosarcoma (RD) cells, each construct yielded comparable expression levels, immunoprecipitation profiles, and avidity for collagen and laminin. However, while RDX2C2 and RDX2C5 transfectants mediated collagen gel contraction, RDX2C4 and a mock transfectant (RDpF) did not. Conversely, only RDX2C4 cells (but not RDX2C2 or RDX2C5) showed enhanced cell migration on collagen and laminin compared with RDpF cells. This indicates markedly differing roles for integrin alpha subunit cytoplasmic domains in post-ligand binding events. Furthermore, stable exertion of physical force (collagen gel contraction) may involve fundamentally different cellular machinery than the transient adhesion occurring during cell migration. Finally, these findings provide insight into a functional flexibility perhaps resulting from multiple integrins binding to identical ligands.

Differential role of lymphocyte function-associated antigens in the activation of nickel-specific peripheral blood T lymphocytes

The possible role(s) of the adhesion molecules LFA-1 alpha (CD11a), LFA-1 beta(CD18), ICAM-1 (CD54), CD2 (T11, LFA-2), and LFA-3 (CD58) in the in vitro activation of nickel-specific peripheral blood (PB) T lymphocytes was studied. For this purpose, monoclonal antibodies (MoAb) to these markers were used. Both LFA-2 and LFA-3 appeared to be consistently involved, whereas LFA-1 was inconsistently involved. In studies using antigen-presenting cells (APC) isolated from peripheral blood to present nickel, anti-LFA-1 alpha and/or LFA-1 beta MoAb partially inhibited the in vitro activation of nickel-specific T lymphocytes in nine of 42 patients allergic to nickel. In the other 33 patients variable results, ranging from a slight increase or inhibition of proliferation to no inhibition at all, was observed, in particular when different anti-LFA-1 alpha MoAb were added to the cultures. In those patients who showed no inhibition when anti-LFA-1 (alpha and beta) MoAb were added, no inhibition was also observed when a mixture of anti-LFA-1 (alpha and beta) and ICAM-1 MoAb were added to the cultures. Similar results were also obtained using epidermal APC. In control experiments the various anti-LFA-1 (alpha and beta) MoAb effectively inhibited the tetanus toxoid and Con-A induced T-lymphocyte proliferation as well as the spontaneous aggregation of the JY cell line. Anti-CD2 and anti-LFA-3 MoAb strongly inhibited the proliferative responses of nickel-specific peripheral blood T lymphocytes from all 42 patients. These results indicated that the receptor-ligand interaction between CD2 and LFA-3 is essential for in vitro activation of nickel-specific peripheral blood T lymphocytes. This activation, however, does not regularly involve LFA-1 molecules on T lymphocytes. The involvement of LFA-1 in the activation of nickel-specific T lymphocytes correlated positively with high patch test scores to nickel and the disease activity in contact dermatitis patients.

Antigen-independent T cell activation mediated by a very late activation antigen-like extracellular matrix receptor

Cell-cell and cell-extracellular matrix (ECM) binding mediated by integrin molecules has been implicated in lymphocyte migration and adhesion. We describe here that ECM binding triggers antigen-independent activation of T cell functions. Fibronectin and vitronectin, when coated on plates, not only acted synergistically on anti-CD3-induced serine esterase release in a murine cytotoxic T lymphocyte clone and interleukin 2 production in a murine helper T cell hybridoma but also could trigger these responses alone. All these stimulatory effects of ECM were abrogated by a monoclonal antibody which reacts with a unique very late activation antigen-like integrin and this monoclonal antibody, when coated on plates, exhibited a similar synergistic effect to that of ECM proteins. Therefore, ECM receptors expressed on activated T cells appear to play an important role in triggering T cell effector functions in localized tissues abundant in ECM proteins.

T cell receptor-independent cell-mediated cytotoxicity by nude mouse lymphokine-activated killer cells

Lymphokine-activated killer (LAK) cells, which can lyse a variety of tumor cells, can be induced from both normal and athymic nude mouse spleen cells by culture with high doses of recombinant interleukin 2 (rIL-2). LAK cells generated from nude mouse spleen cells (Nude-LAK cells) express just Thy 1.2 antigen, but not CD4 and CD8 antigens. Nude-LAK cells express neither T3 molecule, T cell receptor (TCR) alpha beta nor TCR gamma delta on their cell surface. The lack of TCR expression on Nude-LAK cells was confirmed by the results of northern blot analysis. LAK cells generated from normal mouse spleen cells (Nor-LAK) express TCR alpha, beta transcripts, while Nude-LAK cells express only sterile TCR beta transcript, but not TCR alpha transcript. TCR gamma delta transcripts were scarcely detected in both Nor-LAK cells and Nude-LAK cells. Thus, it is strongly suggested that Nude-LAK cells can recognize and lyse tumor cells by TCR-independent mechanisms. Monoclonal antibody against lymphocyte function-associated antigen (LFA-1) molecule can block the cytotoxicity of Nude-LAK cells, indicating an important role of such accessory molecules in Nude-LAK cell-mediated cytotoxicity.

B cell-associated LFA-1 and T cell-associated ICAM-1 transiently cluster in the area of contact between interacting cells

TNP-specific B cells interact with carrier-specific T hybridoma cells in an antigen-specific, MHC-restricted manner. The formation of T cell/B cell conjugates is time and temperature dependent and results in the formation of a broad area of close contact between the interacting cells. In order to determine which surface molecules on the two cells cluster at the interaction site. T cell/B cell conjugates were formalin-fixed at different times following conjugation and were stained with antibodies directed against cell surface molecules. Results of these studies indicate that the alpha- and beta-subunits of LFA-1 on B cells transiently cluster in the area of cell contact. Maximum clustering of LFA-1 occurs at 45 min, after which time LFA-1 redistributes on the surface of the B cells. Several other B cell-associated molecules (MHC Class II, ICAM-1, Ig, B220, J11D, or CD23) do not cluster at the interaction site at any time point. T cell-associated LFA-1 does not cluster with any specific pattern, but ICAM-1 does. Maximum clustering of ICAM-1 occurs 60 to 90 min after intercellular contact. After this time, ICAM-1 redistributes on the surface of the T cells. Although both the alpha- and beta-subunits of LFA-1 cluster at the interaction site on B cells, antibodies recognizing these subunits differ in their ability to affect conjugation. One antibody recognizing the alpha chain of LFA-1 (M17/4.2) inhibits T-cell/B cell conjugation, whereas another antibody that also recognizes the alpha chain-(G-48) enhances conjugation. In contrast, an antibody that recognizes LFA-1 beta (M18/2.a.8) has no effect. An antibody that recognizes ICAM-1 (YN/1.7), the ligand for LFA-1, inhibits conjugation. These data show that, during T cell/B cell interaction. LFA-1 on B cells and ICAM-1 on T cells transiently cluster with similar, albeit not identical, kinetics to the site of cell-cell contact.

ICAM- melanoma cells are relatively resistant to CD3-mediated T-cell lysis

The primary activation pathway of T cells is via the T-cell receptor (TCR)/CD3 complex, which is functionally interrelated with various accessory molecules. We examined the contribution of the lymphocyte-function-associated antigen-I/intercellular adhesion molecule 1 (LFA-1/ICAM-1) interaction to CD3/TCR-mediated lysis by cytotoxic T lymphocytes (CTL). We used ICAM-I-or+ tumor cell lines as target cells and anti-CD3- or anti-LFA-1 containing hetero-cross-linked monoclonal antibody (MAb) to bridge CTL and target cells and simultaneously to activate CTL. The ICAM-1- melanoma-derived cell line IgR39 was relatively resistant to CD3-mediated lysis by both TCR alpha beta + and TCR gamma delta + CTL, when compared with ICAM-1+ cell lines. Induction of ICAM-1 on the membrane of IgR39 cells by tumor necrosis factor (TNF) rendered these cells more susceptible to CD3-mediated lysis. Anti-ICAM-1 MAb inhibited this TNF-enhanced susceptibility to lysis, directly demonstrating that the induction of ICAM-1 was critical in the TNF-induced increase in susceptibility to lysis of IgR39 cells. CTL formed less efficient conjugates with the ICAM-1- cells as compared to ICAM-1+ cells. Both spontaneous and CD3-induced conjugate formation as well as CD3-mediated lysis of ICAM-1- tumor cells by CTL were enhanced by the addition of anti-LFA-1 containing hetero-cross-linked MAb, thereby mimicking the LFA-1/ICAM-1 interaction between CTL and target cells. Soluble anti-CD18 MAb inhibited CD3-mediated lysis of ICAM-1- target cells by CTL without affecting their conjugate formation. Anti-LFA-1 MAb added after conjugate formation still inhibited lysis of both ICAM-1+or- tumor cells. Taken together, these findings suggest that the LFA-1/ICAM-1 interaction co-activates CD3/TCR-mediated lysis by CTL through both an enhanced CTL-target cell binding and the delivery of post-conjugate costimulatory signals.

Adhesion receptors of the immune system

The adhesive interactions of cells with other cells and with the extracellular matrix are crucial to all developmental processes, but have a central role in the functions of the immune system throughout life. Three families of cell-surface molecules regulate the migration of lymphocytes and the interactions of activated cells during immune responses.

Comparative studies of the preparation of immunoliposomes with the use of two bifunctional coupling agents and investigation of in vitro immunoliposome-target cell binding by cytofluorometry and electron microscopy

The two coupling agents SPDP (N-succinimidyl-3-(2-pyridyldithio)propionate) and SATA (N-succinimidyl-S-acetylthioacetate) were compared in their efficiency and feasibility to couple monoclonal antibodies (Abs) via thioether linkage to liposomes functionalized by various lipophilic maleimide compounds like N-(3-maleimidopropionyl)-N2-palmitoyl-L-lysine methyl ester (MP-PL), N-(3-maleimidopropionyl)phosphatidylethanolamide (MP-PE), N6-(6-maleimidocaproyl)-N2-palmitoyl-L-lysine methyl ester (EMC-PL), and N-(6-maleimidocaproyl)phosphatidylethanolamine (EMC-PE). The composition of the liposomes was soy phosphatidylcholine (SPC), cholesterol, maleimide compounds and alpha-tocopherol (1:0.2:0.02:0.01, mol parts), plus N4-oleylcytosine arabinoside (NOAC) as cytostatic prodrug (0.2 mol parts) and a new, lipophilic and highly fluorescent dye N,N'-bis(1-hexylhfetyl)-3,4:9,10-perylenebis(dicarboximid ) (BHPD, 0.006 mol parts). From the maleimide derivatives MP-PL was the most effective in terms of preservation of the coupling activity in dependence of liposome storage. The coupling of the monoclonal A B8-24.3 (mouse IgG2b, MHC class I, anti H-2kb) and IB16-6 (rat IgG2a, anti B16 mouse melanoma) to the drug carrying liposomes was more effective and easier to accomplish with SATA as compared to SPDP. Coupling rates of 60-65% were obtained with SATA at molar ratios of 12 SATA:1 Ab:40 maleimide spacer groups on the surface of one liposome. The highest coupling rates with SPDP were obtained at the ratio of 24 SPDP:1 Ab:40 liposomal maleimide groups, with an Ab binding efficiency of only 20-25%. The optimal in vitro binding conditions to specific target cells (EL4 for B8-24.3-liposomes and B16-F10 for IB16-6-liposomes) were determined by cytofluorometric measurement of the liposomal BHPD fluorescence with SATA linked Abs. Optimal immunoliposome binding to specific epitopes on the target cells was achieved with 1-2 Ab molecules coupled to one liposome, with immunoliposome concentrations of 20-130 nM and with a small incubation volume of 0.3-0.4 ml. The specificity of the binding of B8-24.3-liposomes to EL4 target cells was visualized by scanning electron microscopy. Antibody mediated endocytic uptake of immunoliposomes could be demonstrated by transmission electron microscopy.

Activation of human T cell clones through the UM4D4/CDw60 surface antigen

UM4D4 is a recently defined antigen that is expressed on approximately 25% of peripheral blood T cells, but on the majority of T cells in inflammatory synovial fluid. Anti-UM4D4 activates peripheral blood T cells in the presence of accessory cells and/or phorbol ester. UM4D4 has been assigned to a new antigen cluster termed CDw60. The present study examined the ability of anti-UM4D4 to activate T cell clones derived from the synovial fluid of patients with rheumatoid arthritis. UM4D4 was expressed at varying levels on both lectin-generated and antigen-specific clones, including clones of CD4+, CD8+, and CD4-CD8- phenotypes. Anti-UM4D4 used in soluble form as a single stimulus was typically mitogenic for the CD4+ and some of the CD8+ clones, but not for the CD4-CD8- clones. Phorbol ester boosted the response to anti-UM4D4 in some clones, had no effect in others, and diminished the responses in some cases. In contrast to anti-UM4D4, anti-CD3 was generally not mitogenic in soluble form, although it was mitogenic when conjugated to beads. The data show that T cell clones derived from an inflammatory T cell infiltrate can be readily activated through the UM4D4/CDw60 antigen.

Roles of adhesion molecules in T-cell recognition: fundamental similarities between four integrins on resting human T cells (LFA-1, VLA-4, VLA-5, VLA-6) in expression, binding, and costimulation

This review summarizes our recent work on expression and function of 4 integrins on resting human CD4+ T cells. Three themes are highlighted: multiplicity of molecular pathways of adhesion, regulation of adhesion, and costimulation by adhesion molecules. Four distinct receptor/ligand interactions have been elucidated: LFA-1/ICAM-1, VLA-5/fibronection, VLA-4/fibronectin, and VLA-6/laminin. Our studies indicate fundamental similarities in function and regulation of these four receptor/ligand interactions: 1) acute activation of the T cell (by CD3/TCR crosslinking or by PMA) induces rapid but transient integrin binding function; and 2) higher expression of each integrin on memory T cells compared to naive T cells results in greater binding of memory cells to each ligand. The identification of T-cell integrins (VLA-4, VLA-5, VLA-6) which interact with ECM components directs attention to the potential importance of T-cell interactions with ECM components which either may be immobilized in ECM or which may act as molecular bridges between cells. The existence of multiple adhesion pathways, of multiple ligands for a single receptor (such as LFA-1/ICAM-1 and LFA-1/ICAM-2), multiple receptors for a single ligand (such as VLA-4/FN and VLA-5/FN), and regulation of ligand expression (ICAM-1) provides opportunities for co-operativity, rebundancy and diversity which the T cell utilizes to exquisitely regulate its adhesive interactions. The thesis that adhesion molecules can be multifunctional receptors that also influence signalling is demonstrated by our findings that each of these integrin receptor/ligand interactions is capable of providing a potent costimulatory signal to CD3-mediated T-cell activation. The importance of interactions of T-cell integrins with their cell surface and ECM ligands is discussed with respect to T-cell migration, differentiation and recognition. Analysis of the precise mechanisms by which T cells regulate and exploit these multiple receptor/ligand interactions and the resulting functional consequences of those interactions will be exciting areas of future research.

Expression of the adhesion molecule ICAM-1 and major histocompatibility complex class I antigens on human tumor cells is required for their interaction with autologous lymphocytes in vitro

In a group of 30 human tumors, comprising 12 lung, 14 ovarian, 2 breast carcinomas, 1 hypernephroma and 1 mid-gut carcinoid, the expression of major histocompatibility complex (MHC) class I molecules and the intercellular adhesion molecule 1 (CAM-1, CD54) was found to vary independently. Some tumors expressed both or neither of these molecules. Among 9/13 ICAM-1+ tumors, in which greater than 50% cells reacted with the anti-ICAM-1 monoclonal antibody (mAb) (LB-2), the class I antigen was also detected on greater than 50% of the cells. Only 2 ICAM-1+ tumors were class-I-. In 5/17 cases the tumors were MHC-class-I+ and ICAM-1-. Lymphocytes collected from the blood or from the tumor site were assayed for recognition on the tumor cells in the auto-tumor cytotoxicity test and in mixed lymphocyte tumor cell culture (MLTC). Positive results were obtained only with the MHC-class-I+/ICAM-1+ tumors. In vitro treatment of the tumor cell suspensions with interferon gamma and tumor necrosis factor alpha (TNF alpha) induced or enhanced the ICAM-1 and/or class I antigen expression in 8/12 cases. Of the tumor samples treatged, 8/9 acquired stimulatory capacity and 3/10 became susceptible to lysis by the lymphocytes. In 6/6 MLTC performed with the cytokine-treated tumor cells, cytotoxicity against the autologous tumor was generated. Three of these MLTC lymphocytes also lysed the untreated targets. mAb directed to class I antigens or to ICAM-1 inhibited both the stimulation by and the lysis of tumor cells when confronted with fresh lymphocytes. The cytotoxicity generated in the MLTC was also inhibited. If, however, the cytotoxic function was induced in MTLC containing interleukin-2 (5 U/ml), inhibition was obtained only by pretreatment of the targets with mAb against ICAM-1. The results show thus (a) that the lymphocytes react in vitro with tumor cells only if these express both MHC class I molecules and ICAM-1; (b) that expression of these molecules can be induced by interferon alpha and TNF alpha; (c) that cytotoxic effectors generated in the MLTC with cytokine-treated tumors can also act on the untreated tumor cells. The requirement of the two surface moieties for the interaction with lymphocytes was also substantiated by blockade with relevant mAb.

Costimulatory signal provided by a B-lymphoblastoid cell line and its Ia-negative variant

We have analyzed the requirements of highly purified, resting murine CD4+ T lymphocytes for activation mediated by the lectin Con A and by monoclonal antibodies against the CD3 and Thy-1 molecules. Our results indicate that both the Ia-positive B-lymphoblastoid cell line M12 and its Ia-negative variant M12.C3 can provide the costimulatory activity necessary for these activation pathways. The costimulatory function is preserved upon fixation with paraformaldehyde, indicating that the costimulatory molecule(s) is (are) constitutively expressed on the cell surface. Our experiments also point to interesting differences between the M12 cell line and syngeneic Ia-positive antigen-presenting cells in generating a syngeneic mixed lymphocyte reaction. Finally, we show that the CD4+ T cell-M12.C3 cell interaction can be used to screen for interesting monoclonal antibodies that affect cell function.

The specific interaction of helper T cells and antigen-presenting B cells. IV. Membrane and cytoskeletal reorganizations in the bound T cell as a function of antigen dose

We have used double-immunofluorescence labeling to determine the surface distributions of LFA-1 and CD4, and the intracellular distributions of the cytoskeletal protein talin and of the microtubule organizing center (MTOC) of cloned Th cells in 1:1 cell couples with antigen (Ag)-specific APC of the B cell type (B-APC). The Th cell was directed to a peptide fragment of the Ag OVA in the context of IAd. The B-APC was the transfected A20 B hybridoma cell A20-HL, bearing on its surface a surface Ig specific for the hapten TNP, and pulsed with different concentrations of DNP-OVA. At sufficiently high doses of DNP-OVA (greater than 100 ng/ml), in essentially all couples, LFA-1, CD4, and talin were each concentrated at the Th cell membrane where it was in contact with the B-APC, and the MTOC inside the Th cell was reoriented to face the contact region. At lower doses of DNP-OVA (between 50 and 10 ng/ml), in all couples, LFA-1 and talin were concentrated at the Th/B-APC contact region, but the extent of CD4 clustering, MTOC reorientation, and Th cell proliferation all decreased with decreasing Ag dose. With no Ag, none of these effects was observed. These and other data indicate that two distinct signals are received by the Th cell that is specifically bound to its B-APC. The first signal, at low Ag doses, stimulates a linkage of LFA-1 and talin in the Th cell, and a specific LFA-1-mediated intercellular adhesion; the second signal, at higher Ag doses, is required to induce Th cell proliferation, with which the Th-MTOC reorientation and CD4 clustering are correlated.

Monoclonal antibodies against LFA-1 or its ligand ICAM-1 accelerate CD2 (T11.1 + T11.2)-mediated T cell proliferation

Activation of human-purified T cells can be mediated by pairwise combinations of monoclonal antibodies directed against T11.1 and T11.2 epitopes on the CD2 molecule. Monoclonal antibodies (mAbs) reactive with either the alpha and beta chains of the lymphocyte-function-associated antigen-1 (LFA-1) molecule or one of its ligands, intercellular adhesion molecule-1 (ICAM-1), were found to accelerate anti-CD2-induced proliferation. This effect was seen on thymocytes and resting or preactivated T cells (phytohemagglutinin blasts and alloproliferative T cell clones) and could be observed, following the introduction of anti-LFA-1 or -ICAM-1 mAbs, up to 50 hr after the CD2 stimulatory signal. This effect was equally abrogated by 55 kDa anti-interleukin-2 (IL-2) receptor mAb, but neither the expression of IL-2 receptor nor the production of IL-2 was modified. The effects of anti-LFA-1 or anti-ICAM-1 on T cell activation through the CD2 pathway were therefore opposite to those observed in the CD3 pathway, where both mAbs strongly delayed T cell proliferation.

Leukocyte function-associated antigen 1 is an activation molecule for human T cells

The leukocyte function-associated antigen 1 (LFA-1) molecule is well established as a surface protein involved in cellular adhesion and interaction, but there has been little information about whether engagement of this molecule can also directly modify cellular activation. These studies demonstrate that crosslinking the LFA-1 molecule on human T cell clones transmits a unique signal to the cell. Crosslinking LFA-1 alone did not increase intracellular calcium ([ CA2+]i), nor did crosslinking LFA-1 activate the cells as measured by IL-2 production or [3H]thymidine incorporation. However, when CD3 and LFA-1 were crosslinked, a more prolonged calcium signal was observed than when CD3 alone was crosslinked. Moreover, IL-2 production and DNA synthesis were greatly augmented. These responses could be demonstrated when LFA-1 was crosslinked via either the alpha or the beta chain, and required surface expression of the LFA-1 molecule as no enhancement was observed in T cell clones from a child with leukocyte adhesion deficiency. The enhancement of cellular activation by LFA-1 did not require that it be directly crosslinked to the CD3 complex. Thus, crosslinking LFA-1 alone with isotype-specific secondary antibodies on cells also pretreated with an anti-CD3 mAb of a different Ig isotype stimulated the cells as effectively as crosslinking both surface antigens with GaMIg. Similarly, a delayed, but sustained increase in [Ca2+]i was elicited. This increase in [Ca2+]i and the enhanced functional responses required engagement of CD3 with an intact bivalent anti-CD3 mAb, as crosslinking LFA-1 on cells also reacted with Fab fragments of an anti-CD3 mAb did not increase [Ca2+]i, nor activate the cells. These data indicate that LFA-1 can convey activation signals to T cells. Synergism in signaling can be observed upon crosslinking of LFA-1 and independently crosslinking CD3. In the physiologic interaction between T cells and accessory cells, the interaction of LFA-1 with its ligand, intercellular adhesion molecule 1, may therefore not only facilitate cellular adhesion, but also may amplify T cell activation by delivering costimulatory signals.

Enhanced responsiveness of human memory T cells to CD2 and CD3 receptor-mediated activation

Previous investigations have defined phenotypic differences between unprimed (naive) and antigen-primed (memory) T cells from human peripheral blood. We now report that memory T cells proliferate much more than naive cells when stimulated with anti-CD3 monoclonal antibody or pairs of anti-CD2 monoclonal antibodies. Enhanced responsiveness to receptor-mediated triggering is a novel mechanism for T cells which could facilitate memory cell response to specific antigen. Furthermore, when triggered via either CD2 or CD3, memory T cells produce substantial amounts of interferon gamma while naive cells produce virtually none; this suggests that differentiation from naive to memory state is accompanied by a stable change in regulation of the gene for interferon-gamma. We conclude that naive and memory T cells are dramatically different in function as well as phenotype.

The role of accessory molecules in T-helper activation induced by antigen, lectin, or CD3 antibodies

We examined the role of L3T4 and LFA-1 molecules in T-helper-cell activation, under conditions where the physical stability of T helper-accessory cell interactions was not an issue. T-helper hybridomas were activated by accessory cells coated either with concanavalin A (Con A) or with CD3 antibodies. Activation of the T helper cells was measured by microtubule-organizing center (MTOC) reorientation as an early activation event, and by interleukin-2 (IL-2) production as an indication of a fully matured response. Both parameters were strongly blocked by L3T4 and LFA-1 antibodies in the case of Con A activation. In the case of stimulation with accessory cell-bound CD3 antibody, activation was blocked by LFA-1 but not L3T4 antibody. These results support the notion that L3T4 and LFA-1 molecules play more than a simple adhesion role in T-cell activation. The differential effect of L3T4 antibody in the case of Con A activation vs CD3 activation is consistent with the possibility that L3T4 and the alpha/beta portions of the T-cell receptor must interact during antigen- and lectin-stimulated T-cell activation.

Regulatory properties of LFA-1 alpha and beta chains in human T-lymphocyte activation

Lymphocyte function-associated antigen-1 (LFA-1) is a heterodimer composed of an alpha and beta chain that is expressed on the surface of most leukocytes and is an essential molecule for adhesion reactions between cells participating in the immune response. A putative ligand for LFA-1 is the intercellular adhesion molecule ICAM-1 (refs 3-5). Leukocyte adhesion abnormality is found in patients with LFA-1 deficiency. It is not clear whether binding of ligand to the LFA-1 molecule merely spatially orientates cells towards each other or can also induce signals that regulate cell activation and differentiation. We have recently developed a T-cell proliferation assay which uses immobilized anti-CD3 monoclonal antibodies as stimulant and is independent of LFA-1-mediated cellular adhesion. As there is no interference by anti-LFA-1 monoclonal antibodies with the adhesion-dependent activation steps, this T-cell activation system allows us to investigate whether transmembrane signals are induced by binding of ligand to LFA-1 on T cells. Our data indicate that binding of ligand to LFA-1 results in the transduction of regulatory signal across the plasma membrane, rather like other molecules (CD2, CD4, CD8) (refs 8-11) with signal-modifying properties involved in the adhesion of T cells to target/stimulator cells. Indeed, adhesion molecules might generally be important in signal transduction, even in cells not belonging to the immune system.

Lymphocyte function-associated antigen 1 (LFA-1) and natural killer (NK) cell activity: LFA-1 is not necessary for all killer: target cell interactions

A panel of five monoclonal antibodies detecting human lymphocyte function-associated antigen 1 (LFA-1) was generated and shown by competitive binding studies to react with at least four distinct epitopes on this molecule. The antibodies were then tested for their ability to inhibit the lytic activity of a variety of different human natural killer (NK) populations on a panel of four NK-susceptible target cells (K562, MOLT-4, HSB-2, and Jurkat). When heterogeneous NK populations derived from fresh peripheral blood and mixed-lymphocyte culture (MLC)-generated lines were used, these anti-LFA-1 monoclonal antibodies (MAbs) inhibited lysis of all four NK targets; this finding supports the notion that LFA-1 molecules play an important role in NK-mediated lysis. When tested on a cloned line of NK cells (NK 3.3), lysis of K562 was inhibited by these MAbs, but lysis of the other three targets was not affected. This represents an instance where a MAb specific for LFA-1 inhibits the lytic activity of NK cells against some but not all targets; thus the LFA-1 molecule cannot be considered under all circumstances to be an absolute requirement in NK-mediated lysis.

Involvement of leukocyte function-associated antigen-1 (LFA-1) in the invasion of hepatocyte cultures by lymphoma and T-cell hybridoma cells

We studied the interaction of MB6A lymphoma and TAM2D2 T cell hybridoma cells with hepatocyte cultures as an in vitro model for in vivo liver invasion by these tumor cells. A monoclonal antibody against leukocyte function-associated antigen-1 (LFA-1) inhibited adhesion of the tumor cells to the surface of hepatocytes and consequently strongly reduced invasion. This effect was specific since control antibodies, directed against Thy.1 and against T200, of the same isotype, similar affinity, and comparable binding to these cells, did not inhibit adhesion. This suggests that LFA-1 is involved in the formation of liver metastases by lymphoma cells. TAM2D2 T cell hybridoma cells were agglutinated by anti-LFA-1, but not by control antibodies. Reduction of adhesion was not due to this agglutination since monovalent Fab fragments inhibited adhesion as well, inhibition was also seen under conditions where agglutination was minimal, and anti-LFA-1 similarly affected adhesion of MB6A lymphoma cells that were not agglutinated. The two cell types differed in LFA-1 surface density. TAM2D2 cells exhibited 400,000 surface LFA-1 molecules, 10 times more than MB6A cells. Nevertheless, the level of adhesion and the extent of inhibition by the anti-LFA-1 antibody were only slightly larger for the TAM2D2 cells.

LFA-1 and other accessory molecules functioning in adhesions of T and B lymphocytes

Adhesions of lymphocytes, among themselves or with other cell types, are necessary for most steps in immune responses including both induction and effector phases. Among adhesions of T cells involving specific immunological recognition, CTL-target adhesions have been the most studied. Although CTL-mediated killing is highly specific (specific/nonspecific lytic activity 50-fold), CTL-target adhesion (conjugation) is less so. In the mouse, specificity of conjugation has typically been four to eightfold. Two recent studies with cloned human CTL found much less specificity of conjugation, from one-fold (no specificity) to 1.5-fold. Thus, with cloned human CTL, adhesion may occur promiscuously with any potential target; recognition following adhesion is necessary for lethal hit delivery. The fact that antibodies to the antigen receptor (Ti or CD3) inhibit killing without inhibiting CTL-target conjugation supports this view. The ability of lymphocytes to form nonspecific adhesions, plus the dependence of even the specific mouse adhesions on temperature, metabolic energy, magnesium, and an intact cytoskeleton suggest that the bulk of the strength of T lymphocyte adhesions are not simply the sum of the bonds between antigen receptors (Ti) and antigen. Lymphocytes evidently possess separate "adhesion strengthening" mechanisms. The similarities in the properties of CTL-target adhesions and antigen-independent homotypic B lymphocyte adhesions (Table 2) suggest that at least some of these mechanisms are widely used among cells of hematopoietic origin. MoAbs to most lymphocyte surface molecules, when bound to the living lymphocyte membrane, have no evident functional effects on lymphocyte function. However, a minority can either activate or inhibit lymphocyte functions. Such antibodies identify "leukocyte (or lymphocyte) function-associated antigens," or LFAs (not all of which happen to have "LFA" in their names, Table 1). Most of the inhibitory antibodies inhibit lymphocyte adhesions, and this appears to account for their inhibitory effects on functions such as killing or proliferation. The fact that the binding of antibodies to a particular membrane glycoprotein inhibits adhesion does not guarantee that the glycoprotein in question is a direct participant in adhesion (one of the "glue" molecules). However, there is scanty evidence in support of indirect "negative signals" that may be induced by such antibodies, and direct participation of most LFAs in adhesion seems likely.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction or prevention of immunopathological disease by cloned cytotoxic T cell lines specific for lymphocytic choriomeningitis virus

Cloned lymphocytic choriomeningitis virus (LCMV)-specific cytotoxic T lymphocyte (CTL) lines were prepared from spleens of 129/J (H-2b) mice immunized 7-9 months earlier with LCMV (UBC strain), or of C57BL/10J (H-2b) mice immunized 4 to 5 weeks earlier with LCMV (Armstrong strain). One uncloned and 3 cloned cytotoxic T cell lines were assessed for their respective abilities to produce, or protect against, fatal disease upon transfer to appropriate recipients or to induce specific footpad-swelling reaction. The effects of all lines were essentially identical. In recipient mice acutely infected with LCMV and immunosuppressed either by irradiation (750-990 rds) or treatment with cyclophosphamide, cloned T cells administered intracerebrally (i.c.) caused a convulsive disease and death within 1-4 days. No disease was produced when the same CTL were transferred to uninfected recipients or when they had been frozen and thawed prior to transfer to infected recipients. When admixed with 500 plaque-forming units of LCMV and transferred i.c. to immunocompetent H-2b mice, the T cell clones prevented overt disease. Allogeneic (H-2k) recipients of this same admixture all developed typical LCM disease as did H-2b recipients of the admixture after T cells had been frozen and thawed. Inoculation of cloned CTL into preinfected footpads induced a specific footpad-swelling reaction, which reached maximum levels after about 36 h. Irradiated and infected recipients of cloned LCMV-specific T cells showed the footpad-swelling reaction only when they had been reconstituted with bone marrow cells. In contrast, cloned T cells induced LCM disease in i.c. infected and irradiated mice independent of bone marrow reconstitution. These findings indicate that both fatal LCMV-induced neurologic disease and protection against it are mediated directly by virus-specific CTL.