Measurement of NK activity by the microcytotoxicity assay (MCA): a new application for an old assay.

Natural killer (NK) cells are spontaneously cytotoxic immune effector cells with the ability to selectively destroy tumor cells without harming normal cells. To perform this function, NK cells utilize two main cytotoxicity pathways, the well known perforin/granzyme-mediated secretory/necrotic killing and the recently defined TNF family ligand-mediated non-secretory/apoptotic killing. The former mechanism is manifested mainly against a few cultured leukemia cell targets, while the latter mediates killing against a large variety of tumor cell targets. Therefore, the biological role and significance of these mechanisms might be different. The NK cell-mediated necrotic killing has been reliably and selectively measured in humans by the standard 4-h 51Cr release assay (CRA) against K562 myeloid leukemia cell targets. However, no standardized high throughput assay is available for testing the NK cell-mediated apoptotic killing. Here, we introduce the modified MCA as a convenient method for measuring perforin/granzyme-independent NK cell-mediated apoptotic killing. The assay is performed in microwells of Terasaki tissue culture microtest plates, using adherent tumor cell targets, which are selectively susceptible to non-secretory/apoptotic killing and resistant to secretory/necrotic killing mediated by NK cells. Target cells are plated in microwells and incubated overnight to adhere to the plastic surface and to regenerate cell surface-bound TNF family receptors. Following this adherence, target cells are co-incubated with freshly isolated human peripheral blood mononuclear leukocytes (PBMNL) or purified subpopulations of immune cells for 24 h in various effector/target (E/T) ratios. During this incubation, dead target cells become non-adherent and are removed by washing the wells. Remaining adherent (viable) target cells are fixed, stained and optically counted. A notable dose-dependent (peak at 200:1 E/T ratio), time-dependent (peak at 24 h of incubation) and donor-dependent killing of tumor cells was consistently and reproducibly induced by PBMNL of normal donors. Using purified subpopulations of immune cells, it was demonstrated that among PBMNL, CD3(-)CD56(+)CD16(+) mature NK cells are the only mediators of tumor cell killing in MCA, as well as in CRA. Comparative studies of NK activity detected by MCA and CRA, performed with PBMNL from normal individuals and breast cancer patients, showed no significant correlation between the cytotoxicities measured in the two assays. In addition, while NK activity measured in CRA was normal in most breast cancer patients, NK activity assessed in MCA was decreased in a large majority of the patients. Thus, MCA is a sensitive NK assay, which is biologically different from CRA, and may be clinically relevant. MCA has also a higher throughput, and is more practical and economical than CRA.

Immune response against 3LL Lewis lung carcinoma potentiates the therapeutic efficacy of endostatin.

The ability of the antitumor immune response to potentiate the therapeutic efficacy of the antiangiogenic agent endostatin was investigated. The antitumor effects of endostatin were tested against weakly immunogenic 3LL Lewis lung carcinoma and its highly immunogenic variant 3LL-C75. Using in vivo Matrigel assay, it was found that the recombinant endostatin produced in the authors' laboratory has a potent antiangiogenic effect. Endostatin manifested a more potent antitumor effect against highly immunogenic 3LL-C75 than weakly immunogenic 3LL tumor. Endostatin induced regression of immunogenic 3LL-C75 tumor in 40% of C57BL/6 mice, whereas partial inhibition and no regression were found in mice bearing weakly immunogenic 3LL tumor. 3LL and 3LL-C75 cells produced similar amounts of Vascular Endothelial Growth Factor, and immunohistochemical analysis revealed that endostatin treatment reduced microvessel density in both 3LL and 3LL-C75 tumors. However, infiltration of T lymphocytes was observed in 3LL-C75 but not in 3LL tumors. These results suggest that the host's immune response may potentiate the antitumor effects of antiangiogenic agents. This possibility was further supported by findings that the antitumor activity of endostatin against 3LL-C75 tumor was lower in immunodeficient than in immunocompetent mice. Stimulation of immune response against 3LL tumor by vaccination with highly immunogenic 3LL-C75 cells substantially increased the antitumor effect of endostatain, resulting in a complete and permanent regression of 3LL tumor in 50% of mice. Tumor vaccination or endostatin treatment applied separately inhibited but did not induce regression of 3LL tumor. These results suggest that the combined attack against tumor cells and the tumor vascular system using antitumor immune mechanisms and antiangiogenic drugs can be a promising strategy for cancer treatment.

Ig-binding receptors on human NK cells as effector and regulatory surface molecules.

The receptors on human natural killer 9NK cells which can specifically bind the Fc portion of immunoglobulin molecules (Fc receptors) have been extensively studied. The best known and studied Fc receptor on human NK cells is FcgammaRIIIa. Interactions of NK cells with IgG antibodies via this receptor are well known to induce a signal transduction cascade and lead to antibody-dependent cell-mediated cytotoxicity (ADCC) as well as release of various cytokines. In addition, interactions with monomeric IgG and FcgammaRIIIa have been demonstrated, which result in negative regulation of NK activity and other immunomodulatory effects. Over the past several years, it has also become increasingly appreciated that human NK cells express a variety of other Fc receptors, including FcmuR, which also can mediate effector and immunoregulatory functions. Also, a novel form of FcgammaR has been demonstrated on human NK cells, termed FcgammaRIIc. Recent molecular studies have shown considerable polymorphism in the genes for FcgammaIIc and the functional consequences are being dissected. This appears to include cross-talk between FcgammaRIIIa and at least some forms of FcgammaRIIc, which may have important functional consequences.

Therapeutic activity of NK cells against tumors.

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

Pre-transplant immunological profile and risk factor analysis of post-transplant lymphoproliferative disease development: the results of a nested matched case-control study. The University of Pittsburgh PTLD Study Group.

Development of post-transplant lymphoproliferative disease (PTLD) is a major complication of organ transplantation. While immune mechanisms seem to play a major role in the development of PTLD, how the immune system contributes to the process of PTLD development or its regression remains unknown. Between 1990-1994, 303 organ transplant recipients were enrolled into a prospective study designed to analyze risk factors for PTLD. Using a nested case-control design, 9 PTLD and 18 control patients were matched for age, EBV serological status at the time of transplantation, and, in most cases, for the type of transplanted organ. The immunologic profiles of both groups were compared prior to and following transplantation. Immune measures included absolute numbers of lymphocytes and of subsets of T, B and natural-killer (NK) cells as well as spontaneous NK-cell and in vitro generated LAK-cell activities. A consistent trend for higher levels at baseline as well as following transplantation for almost all immune parameters was observed in patients who developed PTLD. A high absolute count of activated NK cells (CD56+ DR+) at baseline was found to be a significant predictor of PTLD development. The immunologic profile of patients who developed PTLD was consistent with pre- as well as post-transplant chronic immunologic stimulation, and not immunosuppression. In the PTLD group, 3 patients had pre-transplant autoimmune hepatitis and one had primary biliary cirrhosis, which suggests that the underlying presence of certain autoimmune disorders in organ transplant recipients might predispose to PTLD development.

Constitutive expression and role of the TNF family ligands in apoptotic killing of tumor cells by human NK cells.

Natural killer cells mediate spontaneously secretory/necrotic killing against rare leukemia cell lines and a nonsecretory/apoptotic killing against a large variety of tumor cell lines. The molecules involved in nonsecretory/apoptotic killing are largely undefined. In the present study, freshly isolated, nonactivated, human NK cells were shown to express TNF, lymphotoxin (LT)-alpha, LT-beta, Fas ligand (L), CD27L, CD30L, OX40L, 4-1BBL, and TNF-related apoptosis-inducing ligand (TRAIL), but not CD40L or nerve growth factor. Complementary receptors were demonstrated to be expressed on the cell surface of solid tumor cell lines susceptible to apoptotic killing mediated by NK cells. Individually applied, antagonists of TNF, LT-alpha1beta2, or FasL fully inhibited NK cell-mediated apoptotic killing of tumor cells. On the other hand, recombinant TNF, LT-alpha1beta2, or FasL applied individually or as pairs were not cytotoxic. In contrast, a mixture of the three ligands mediated significant apoptosis in tumor cells. These findings demonstrate that human NK cells constitutively express several of the TNF family ligands and induce apoptosis in tumor cells by simultaneous engagement of at least three of these cytotoxic molecules.

Ligand binding specificities and signal transduction pathways of Fc gamma receptor IIc isoforms: the CD32 isoforms expressed by human NK cells.

We recently reported that human NK cells express, in addition to CD16 [Fcgamma receptor (FcgammaR) IIIA], a second type of FcgammaR, namely CD32 (FcgammaRII). Molecular characterization of CD32 transcripts expressed by highly purified NK cells revealed that they predominantly express products of the FcgammaRIIC gene. Using stable Jurkat transfectants we have analyzed the functional properties of two FcgammaRIIc-specific isoforms isolated from NK cells, namely FcgammaRIIc1 and FcgammaRIIc3, which differ in their cytoplasmic tails. The ligand binding specificity for both murine and human IgG isotypes was found to be similar to that observed for FcgammaRIIb isoforms. Immunoprecipitation studies of FcgammaRIIc isoforms expressed in Jurkat cells revealed a protein of around 40 kDa for FcgammaRIIc1, and a protein of around 32 kDa for FcgammaRIIc3. Signal transduction studies performed on FcgammaRIIc1-expressing Jurkat cells indicated that this molecule is functional, i. e. capable of Ca2+ mobilization and activation of Lck, Zap-70 and Syk protein tyrosine kinases, although the CD3 zeta chain was not found to functionally associate with FcgammaRIIc1. In contrast, FcgammaRIIc3 transfectants showed an impaired ability of this molecule to mobilize Ca2+, but activation of Lck was detected following activation via FcgammaRIIc3. These studies demonstrate the functional activity of FcgammaRIIc isoforms and suggest that the presence of CD32, in addition to CD16, on NK cells may have functional relevance.

Characterization of the stage in natural killer cell development in 14.5-day mouse fetal liver using adult bone marrow stroma.

Nonstimulated fetal liver (FL) from 14.5-day gestation mice had no natural killer (NK) cell activity and <3% expressed NK1.1. Even after short-term (3-4 day) culture of FL with the late-acting cytokines, interleukin (IL)-15 or IL-2, little or no NK activity was detected. However, longer-term (13 day) culture with IL-2 plus stroma derived from bone marrow (BM) of adult mice, resulted in extensive proliferation and differentiation to mature NK cells. Cell numbers began to increase after 4 days, and by day 13, they had increased 40-fold and 69% of the cells were NK1.1+ with high NK activity and 5%-10% were NK1.1- B220+. With stroma, but no IL-2, equivalent proliferation occurred, but differentiated cells were predominantly NK1.1- B220+, not NK cells. Culture for 13 days without stroma, but with either IL-2, IL-15, FLTK3-ligand (L) or stroma-conditioned medium, resulted in less than fivefold expansion, and minimal NK activity. Culture with combinations of FLTK3-L or ckit-L plus IL-15 or IL-2 increased both cell number and NK activity, but the increase in cell number was less than that seen with stroma plus IL-2. By limiting dilution assay on stroma plus IL-2, the precursor frequency was 1/(2660+/-292) whole FL cells and the absolute number, but not the frequency, increased during culture on stroma without IL-2. The NK cell progenitors were found in sorted NK1.1- and Sca-1+ c-kit+ lineage- subpopulations at a frequency of 1/(156+/-52.5). Together, these data suggest that the NK lineage cells in FL are primarily in early stages of development. They are highly proliferative, respond to early acting cytokines and express stem cell markers.

Expression of murine CD34 by fetal liver NK cell progenitors.

Although 14.5-day murine fetal liver (FL) has few, if any, mature natural killer (NK) cells, culture of FL with recombinant human IL-2 (rhIL-2) and stroma from irradiated NK longterm bone marrow cultures (NK-LTBMC) allows proliferation and differentiation of NK cell progenitors. Using this system, NK cell progenitors were found in both CD34+ and CD34- sorted subpopulations of FL. The CD34 antigen was expressed by 14+/-1.3% of whole FL cells, while mature NK cells cultured from NK cell precursors in FL did not express the CD34 antigen. Anti-TER-119 mAb reacted with 84%+/-10.3% of the FL cells, and NK cell progenitors were enriched in the TER-119- subpopulation. After coculture with rhIL-2 and stroma, neither TER-119- nor TER-119+ cells expressed antigens associated with T cells (CD3, CD4, and CD8) or myeloid cells (Gr-1 and Mac-1). Only the TER-119 subpopulation generated NK1.1+ (77%) and B220+ (87%) cells. Within the TER-119 subpopulation, both CD34+ and CD34- cells generated cytolytic and NK1.1+ cells after culture. By a limiting dilution assay (LDA) of the Lin (i.e., negative for NK1.1, CD3, CD4, CD8, B220, Gr-1, and TER-119) CD34 positive or negative subpopulations, the calculated mean frequency of NK cell progenitors was about 1/100 for the CD34+Lin- subpopulation and about 1/(200-300) for the CD34-Lin- subpopulation. In kinetic studies, we found that NK1.1 antigen expression continued to increase with time in culture for both the CD34+Lin- and CD34-Lin- fractions. In contrast, the percentage of CD34+ cells decreased rapidly and produced CD34- cells, and the CD34- population remained CD34-. These data suggest that both CD34+ and CD34- subpopulations of FL can differentiate into NK cells when cocultured for 13 days with irradiated NK-LTBMC stroma and rhIL-2, and that CD34+ progenitors differentiate to CD34- precursors, which in turn differentiate to CD34- mature NK cells.

Dysfunction of natural killer activity in a family with chronic fatigue syndrome.

A family was identified with 5 of 6 siblings and 3 other immediate family members who had developed chronic fatigue syndrome (CFS) as adults. All 8 met criteria for the CFS case definition as recommended by the Centers for Disease Control and Prevention. Sixty-eight blood samples were obtained over a period of 2 years from 20 family members (8 affected, 12 unaffected) and 8 normal controls. All blood samples were tested for NK activity in 4-h 51Cr-release assays and for the number of circulating CD3-CD56(+) and CD3-CD16(+) by flow cytometry. NK activity of the affected immediate family members (cases, n = 8) was significantly lower (P = 0.006, two-sided) than that of the concurrently tested normal controls. The results for unaffected family members were intermediate between these two groups, and the pairwise comparison of unaffected family members to either cases or controls showed no statistically significant difference (P = 0.29, two-sided). No differences were seen between the groups in the absolute number of CD3-CD56(+) or CD3-CD16(+) lymphocytes in the peripheral blood. Familial CFS was associated with persistently low NK activity, which was documented in 6/8 cases and in 4/12 unaffected family members. In the family with 5 of 6 siblings who had documented CFS, 2 of their offspring had pediatric malignancies. Low NK activity in this family may be a result of a genetically determined immunologic abnormality predisposing to CFS and cancer.

Biological properties of recombinant alpha-interferons: 40th anniversary of the discovery of interferons.

IFNs were first described as potent antiviral agents 40 years ago, and recombinant IFN-alpha2a and IFN-alpha2b were approved for the treatment of hairy cell leukemia just 11 years ago. Today, alpha-IFNs are approved worldwide for the treatment of a variety of malignancies and virologic diseases. Although the exact mechanism of action of IFN-alpha in the treatment of such diseases is not fully understood, many advances have been made in the characterization of the physicochemical and diverse biological properties of this highly pleiotropic cytokine. Here we review recent developments in our understanding of the antiviral and immunoregulatory properties of IFN-alpha, the nature of the multisubunit IFN-alpha receptor, and the molecular mechanisms of signal transduction. Where available, we have included comparative data on recombinant alpha-IFNs derived from both naturally occurring and nonnaturally occurring synthetic genes. We also review clinical data and data on the side effects and antigenicity of different sources of recombinant alpha-IFNs in humans. These latter topics are of clinical interest, because they may potentially affect the efficacy of these various products. Hopefully, what is already known about IFN will prompt further exploration into the mechanism(s) of action of IFN-alpha and thus deliver new applications for this prototypic cytokine, whose full therapeutic potential is yet to be realized.

Stable transduction of the interleukin-2 gene into human natural killer cell lines and their phenotypic and functional characterization in vitro and in vivo.

A variety of strategies have been attempted in the past to stably transduce natural killer (NK) cells with cytokine or other cellular genes. Here, we demonstrate the successful delivery of the interleukin-2 (IL-2) gene into two human NK cell lines, IL-2-dependent NK-92 and IL-2-independent YT, by retroviral transduction. An MuLV-based retroviral vector expressing human IL-2 and neor markers from a polycistronic message was constructed and transduced into a CRIP packaging cell line. By coincubation of NK cells with monolayers of CRIP cells or by using retrovirus-containing supernatants in a flow-through method, 10% to 20% of NK cells were stably transduced. Upon selection in the presence of increasing G418 concentrations, transduced NK cells were able to proliferate independently of IL-2 for more than 5 months and to secrete up to 5.5 ng/10(6) cells/24 h of IL-2. IL-2 gene-transduced NK-92 cells had an in vitro cytotoxicity against tumor targets that was significantly higher than that of parental cells and secreted interferon gamma (IFNgamma) and tumor necrosis factor alpha (TNFalpha) in addition to IL-2. Moreover, the in vivo antitumor activity of IL-2 gene-transduced NK-92 cells against established 3-day liver metastases in mice was greater than that of parental nontransduced NK cells. Stable expression of the IL-2 transgene in NK cells improved their therapeutic potential in tumor-bearing hosts. Thus, transduced NK cells secreted sufficient quantities of bioactive IL-2 to proliferate in vitro and mediated the antitumor effects both in vitro and in vivo in the absence of exogenous IL-2. These results suggest that genetic modification of NK cells ex vivo could be useful for clinical cancer therapy in the future.

Natural killer cells and tumor therapy.

Evidence has been reviewed which indicates that NK cells play a role in the control of metastasis dissemination. Both activation of endogenous NK cells in a tumor-bearing host and adoptive transfer of ex vivo activated NK cells may be therapeutically beneficial. The small number of phase I/II clinical trials of AIT with A-NK cells performed in patients with cancer so far does not allow firm conclusions, except to ascertain the feasibility and a lack of toxicity of this form of therapy. Although numerous trials have been performed with BRMs, many of which are known to upregulate NK activity in vivo, a general lack of correlations between clinical responses or survival and upregulated NK activity in the peripheral blood has dampened enthusiasm for biological therapies. However, these clinical trials have been confined largely to patients with advanced metastatic disease. It is highly likely that tumor-induced immunosuppression plays a crucial role in neutralizing the benefits of BRM therapy, and that levels of effector cell activation sufficient for metastasis elimination are seldom achieved in this clinical setting. On the other hand, administration of BRMs in the adjuvant setting could be more effective and when combined with monitoring for effector cell functions might perhaps provide a better guide for achieving the levels of endogenous NK activity necessary for elimination of remaining or occult metastases. An improved understanding of NK cell biology in cancer patients is likely to serve as a positive reinforcement for design of a new generation of clinical trials incorporating novel approaches to NK cell mediated cancer therapy.

Expression of functional CD32 molecules on human NK cells is determined by an allelic polymorphism of the FcgammaRIIC gene.

Human natural killer (NK) cells were thought to express only FcgammaRIIIA (CD16), but recent reports have indicated that NK cells also express a second type of FcgammaR, ie, FcgammaRII (CD32). We have isolated, cloned, and sequenced full-length cDNAs of FcgammaRII from NK cells derived from several normal individuals that may represent four different products of the FcgammaRIIC gene. One transcript (IIc1) is identical with the already described FcgammaRIIc form. The other three (IIc2-IIc4) appear to represent unique, alternatively spliced products of the same gene, and include a possible soluble form. Analyses of the full-length clones have revealed an allelic polymorphism in the first extracellular exon, resulting in either a functional open reading frame isoform or a null allele. Stable transfection experiments enabled us to determine a unique binding pattern of anti-CD32 monoclonal antibodies to FcgammaRIIc. Further analyses of NK-cell preparations revealed heterogeneity in CD32 expression, ranging from donors lacking CD32 expression to donors expressing high levels of CD32 that were capable of triggering cytotoxicity. Differences in expression were correlated with the presence or absence of null alleles. These data show that certain individuals express high levels of functional FcgammaRIIc isoforms on their NK cells.

Conservation of total T-cell counts during HIV infection: alternative hypotheses and implications.

While CD4+ T-cell counts in the blood of HIV-infected individuals gradually decrease, there is a parallel increase in the number of blood CD8+ T cells such that the total number of T cells remains essentially constant for several years (1). The basis and significance of this phenomenon are not known. Based on a statistical analysis of longitudinal T-cell counts from the Transfusion Safety Study (TSS) database and on theoretical considerations, we evaluate several alternative models, including versions of the "blind homeostasis" (BH) hypothesis (1-3). At issue is the nature of the homeostatic regulation of lymphocytes and its apparent failure in HIV infection. The most plausible explanation for the conservation of total blood T-cell numbers while subset ratios change is that CD4+ and CD8+ T cells compete for a limited access to the blood compartment. Such interaction between the subsets implies, in particular, that changes in the number of CD4+ T cells occurring in other tissues cannot be reliably inferred from those observed in the blood. We reiterate propositions made earlier (4) that much of the apparent "depletion" of CD4+ lymphocytes during the asymptomatic phase of HIV infection may be attributed to redistribution between the tissues and the blood compartment.

Review of recent developments in the molecular characterization of recombinant alfa interferons on the 40th anniversary of the discovery of interferon.

Recombinant alfa interferons (IFN-alpha s) are approved worldwide for the treatment of a variety of cancers and diseases of virologic origin. A series of recent advances in the molecular characterization of recombinant IFN-alpha s have allowed the determination of the three-dimensional IFN-alpha 2b structure by high-resolution x-ray crystallography. We review here recent developments in our understanding of the molecular and physicochemical properties of recombinant IFN-alpha, including our current state of knowledge of the IFN-alpha gene family and the multiple species of human leukocyte IFN. Based on the reported three-dimensional structure of IFN-alpha 2b, we propose a molecular model for the IFN-alpha 2b receptor complex and predict models for the naturally occurring subtypes IFN-alpha 1 and IFN-alpha 8, as well as the synthetic, non-naturally occurring consensus IFN. Such models provide molecular insights into the mechanism of action of IFN-alpha.

Comparison of toxicity and survival following intraperitoneal recombinant interleukin-2 for persistent ovarian cancer after platinum: twenty-four-hour versus 7-day infusion.

PURPOSE: To compare the toxicity, pharmacokinetics, and efficacy seen in ovarian cancer patients treated with escalating doses of intraperitoneal (I.P.) interleukin-2 (IL-2) by two different infusion schedules. PATIENTS AND METHODS: Forty-five patients were sequentially entered onto a phase I/II study in groups of four at fixed dosage tiers of 6 x 10(4), 6 x 10(5), 6 x 10(6), and 3 x 10(7) IU/m2/d in either of two schedules: (A) intermittent weekly infusions of 24 hours' duration; or (B) alternating continuous 7-day infusions followed by 7-day intervals without therapy. Eligibility criteria included > or = six courses of prior platinum-based chemotherapy and laparotomy-confirmed persistent or recurrent ovarian cancer. RESULTS: Forty-one eligible patients received I.P. IL-2 and were assessable for toxicity, but six patients were not assessable for response, which left 35 patients assessable for response. Significant locoregional dose-limiting toxicity was seen with the 7-day infusions (including bowel perforation), with 600,000 IU/m2 as the maximum-tolerated dose (MTD), but catheter infection was the only significant complication seen with the 24-hour infusions, for which an MTD was not established. Among 35 assessable patients, there were six laparotomy-confirmed complete responses (CRs) and three partial responses, for an overall response rate of 25.7% (nine of 35). The median survival time of the cohort was 13.7 months and the overall 5-year survival probability was 13.9%. For the nine patients who demonstrated responses (six on the 24-hour infusion and three on the 7-day infusion), the median survival time has not been reached (range, 27 to 90+ months). CONCLUSION: I.P. IL-2 is better tolerated as a weekly infusion as compared with a 7-day infusion and demonstrates evidence of possible long-term efficacy in a modest number of patients. A randomized trial is indicated to determine if the prolonged survival seen in this study is a due to I.P. IL-2 therapy or other factors that cannot be controlled for in a single-arm study.