Secondary malignancies in chronic myeloid leukemia patients after imatinib-based treatment: long-term observation in CML Study IV.

Treatment of chronic myeloid leukemia (CML) has been profoundly improved by the introduction of tyrosine kinase inhibitors (TKIs). Long-term survival with imatinib is excellent with a 8-year survival rate of ∼88%. Long-term toxicity of TKI treatment, especially carcinogenicity, has become a concern. We analyzed data of the CML study IV for the development of secondary malignancies. In total, 67 secondary malignancies were found in 64 of 1525 CML patients in chronic phase treated with TKI (n=61) and interferon-α only (n=3). The most common malignancies (n⩾4) were prostate, colorectal and lung cancer, non-Hodgkin's lymphoma (NHL), malignant melanoma, non-melanoma skin tumors and breast cancer. The standardized incidence ratio (SIR) for all malignancies excluding non-melanoma skin tumors was 0.88 (95% confidence interval (0.63-1.20)) for men and 1.06 (95% CI 0.69-1.55) for women. SIRs were between 0.49 (95% CI 0.13-1.34) for colorectal cancer in men and 4.29 (95% CI 1.09-11.66) for NHL in women. The SIR for NHL was significantly increased for men and women. An increase in the incidence of secondary malignancies could not be ascertained. The increased SIR for NHL has to be considered and long-term follow-up of CML patients is warranted, as the rate of secondary malignancies may increase over time.

Long-term outcome of patients with newly diagnosed chronic myeloid leukemia: a randomized comparison of stem cell transplantation with drug treatment.

Tyrosine kinase inhibitors represent today's treatment of choice in chronic myeloid leukemia (CML). Allogeneic hematopoietic stem cell transplantation (HSCT) is regarded as salvage therapy. This prospective randomized CML-study IIIA recruited 669 patients with newly diagnosed CML between July 1997 and January 2004 from 143 centers. Of these, 427 patients were considered eligible for HSCT and were randomized by availability of a matched family donor between primary HSCT (group A; N=166 patients) and best available drug treatment (group B; N=261). Primary end point was long-term survival. Survival probabilities were not different between groups A and B (10-year survival: 0.76 (95% confidence interval (CI): 0.69-0.82) vs 0.69 (95% CI: 0.61-0.76)), but influenced by disease and transplant risk. Patients with a low transplant risk showed superior survival compared with patients with high- (P<0.001) and non-high-risk disease (P=0.047) in group B; after entering blast crisis, survival was not different with or without HSCT. Significantly more patients in group A were in molecular remission (56% vs 39%; P=0.005) and free of drug treatment (56% vs 6%; P<0.001). Differences in symptoms and Karnofsky score were not significant. In the era of tyrosine kinase inhibitors, HSCT remains a valid option when both disease and transplant risk are considered.

Response to pentostatin in hairy-cell leukemia refractory to interferon-alpha. The European Organization for Research and Treatment of Cancer Leukemia Cooperative Group.

Interferon-alpha (IFN-a) or 2'-deoxycoformycin (pentostatin; DCF) have each been shown to be highly active in hairy-cell leukemia (HCL). In this phase II study of the Leukemia Cooperative Group of the European Organization for Research and Treatment of Cancer (EORTC), the efficacy and toxicity of DCF were investigated in patients who were resistant to IFN-a treatment. Resistance was defined as: (1) progressive disease (PD) under IFN-a therapy for more than 2 months; (2) stable disease (SD) after more than 6 months of IFN-a treatment; (3) relapse within 3 months of discontinuing IFN-a; and (4) intolerance to IFN-a because of World Health Organization (WHO) grade 3 or 4 toxicity. DCF was applied at a dosage of 4 mg/m2 weekly x 3, then 4 mg/m2 every other week x 3. Responders were given a maintenance therapy once per month for a maximum of 6 months. At the time of report, 33 patients with resistant disease were evaluable for response and toxicity. Median duration of IFN-a therapy before DCF administration was 14.7 months (range, 1 to 41 months). Complete remissions (CRs) were achieved in 11 patients and partial remissions (PRs) in 15, resulting in a total response rate of 78.8%. Median interval between beginning of DCF therapy to best response was 3.9 months with a range from 2.0 to 7.0 months. Two patients who achieved PR have relapsed 7 and 14 months after cessation of DCF therapy. The median duration of response was over 11.5 months (range, over 3.0 to over 24.0 months). Three patients died within the first 6 weeks of DCF treatment: one of drug-unrelated cardiomyopathy and two of fungal pneumonia. The patients with early death (n = 3) and nonresponsive disease (n = 4) received IFN-a treatment for a longer period (median, 18.0 months) than did the 26 responsive patients (median, 10.0 months). Major side effects included nausea, skin rash, and infections and were otherwise mild. Thus, DCF is highly active in patients with HCL resistant to IFN-a.

Recovery of immune functions in dogs after total body irradiation and transplantation of autologous blood or bone marrow cells.

The restoration of immune functions was followed in dogs for 101 days after fractionated total body irradiation and autologous transfusion of peripheral blood leukocytes (PBL) or bone marrow (BM) cells. Median numbers of 0.9 X 10(5) granulocyte-macrophage progenitor cells per kilogram of body weight were transferred in either group of recipients. The following parameters recovered more rapidly in PBL recipients as opposed to BM recipients: total blood lymphocyte, T- and B-cell counts, serum levels of immunoglobulins IgM and IgA, in vitro blastogenic responses after stimulation with concanavalin A and pokeweed mitogen, and in vitro plasma cell formation after polyclonal B-cell activation with pokeweed mitogen with or without lipopolysaccharide. No major differences were noted for the restoration of serum IgG levels. Circulating lymphocyte and T-cell numbers remained subnormal for more than three months in both groups, whereas B-cell numbers and serum levels of IgA continued to be depressed in BM recipients only. Thus, autologous PBL restored immune functions more rapidly than did BM. Transplantation of PBL, alone or in addition to autologous BM, might also shorten the period of immunodeficiency after cytoreduction in a variety of malignancies in man.

Progenitor cell (CFUc) reconstitution after autologous stem cell transfusion in lethally irradiated dogs: decreased CFUc populations in blood and bone marrow correlate with the fraction mobilizable by dextran sulphate.

The present study in dogs concerns the functional potentials of the myeloid progenitor cell system during hemopoietic recovery after lethal total-body irradiation and autotransfusion of cryopreserved stem cells derived from peripheral blood and bone marrow. CFUc were assayed in peripheral blood and bone marrow before and at various intervals after grafting. In addition, the net increase of CFUc mobilizable from extravascular sites into the blood was determined after i.v. injection of 15 mg/kg body weight of dextran sulphate (DS). The data indicate a sustained deficiency of CFUc in otherwise hematologically normal recipients. The DS-response was subnormal even 225 days after transplantation, suggesting residual damage of the hemopoietic system is present for a prolonged period of time. A significant correlation was found between the actual blood CFUc concentration and the numbers of CFUc mobilizable by DS. Insight into progenitor cell kinetics and corresponding changes in the size of a mobilizable pool in extravascular sites may be helpful in estimating the marrow reserve of transplanted individuals suffering myelosuppressive effects of chemotherapy.

Canine blood mononuclear cells mediate contact-dependent impairment of granulocyte-macrophage colony formation by bone marrow cells.

Nonphagocytic, nonadherent mononuclear cells from canine peripheral blood (PBMC) were shown to suppress colony formation in agar of autologous and allogeneic bone marrow granulocyte-macrophage progenitor cells (CFU-GM). Suppression required previous cell-to-cell contact in liquid culture between PBMC and bone marrow cells (BMC) and was time- and dose-dependent and resistant to x-irradiation with 20 Gy. Small BMC were less susceptible than large BMC, whereas day-7 and day-14 CFU-GM were equally suppressed. Cryo-preservation of BMC did not enhance CFU-GM inhibition. Spontaneous inactivation during liquid culture of CFU-GM or accessory cells by PBMC is the likely effector mechanism. Possible recognition structures are different from dog leukocyte antigens A and B. Canine PBMC or subpopulations thereof might participate in the regulation of normal hemopoiesis and in the rejection of hemopoietic stem cell grafts by natural killer cell-like mechanisms as has been suggested for human and murine natural killer cells.

Anti-interferon-gamma antibodies in a patient undergoing interferon-gamma treatment for systemic mastocytosis.

An 81-year-old woman with systemic mastocytosis responded to subcutaneous recombinant interferon-gamma (rIFN-gamma) treatment for about 6 months, when intestinal symptoms gradually recurred. A serum sample obtained 3 months later was positive for specific rIFN-gamma-binding antibodies, which had been absent at the initiation of treatment. Cessation of IFN-gamma therapy was followed by a slow decline of IFN antibody titers. The IFN-gamma antibodies were of polyclonal or oligoclonal origin, with a predominance of IgG1 and IgG2 and small amounts of IgA and IgM. They neutralized the antiviral activity of both rIFN-gamma and, less efficiently, natural IFN-gamma in vitro. The time course of the neutralizing titers paralleled the IFN-binding activity of the antibodies. Thus, like other cytokines, rIFN-gamma may be immunogenic in rare patients and elicit the formation of neutralizing antibodies that may impair the therapeutic activity of the drug and interfere with the endogenous IFN-gamma system.

Sensitive antiproliferative neutralization assay for the detection of neutralizing IFN-alpha and IFN-beta antibodies.

Antibodies to interferon (IFN) may compromise IFN treatment in some patients. In tumor therapy, a critical function of type I IFNs is their antiproliferative effect. For the quantification of neutralizing IFN antibodies we have developed an antiproliferative neutralization assay (APA) based on the reduction of IFN-mediated growth inhibition of Daudi cells by IFN-alpha and IFN-beta antibodies. Proliferation was quantified by [3H]thymidine incorporation, and the neutralizing potency of IFN antibody-positive sera was expressed as the neutralizing titer inhibiting 50% of the antiproliferative activity of 10 IU/ml of IFN (NT50). The APA is easy to perform, reproducible, and more sensitive than a well-established antiviral neutralization assay (AVA). All 30 sera with recombinant IFN-alpha 2a-binding antibodies proved to be neutralizing antibody-positive in the APA whereas seven were scored antibody-negative or uninterpretable in the AVA. The APA is recommended as a second or third line assay for the estimation of the neutralizing potency of spontaneous or treatment-induced IFN-alpha and IFN-beta-specific antibodies.

In vitro induction of B cell differentiation in canine mononuclear blood cells: bacterial lipopolysaccharide modulates the action of pokeweed mitogen.

Pokeweed mitogen (PWM) was shown to induce the generation of plasmacytoid cells (PC) from canine peripheral blood mononuclear cells in vitro. PC were detected by immunofluorescence staining of cytoplasmic immunoglobulin. Optimal culture conditions for PC formation were established and the range of the PC response in normal dogs assessed. Addition to PWM of bacterial lipopolysaccharide enhanced PC formation in most instances. This occurred in the absence of increased cell proliferation and without altering the time course of the response. Mitogen-induced PC generation may provide a useful tool for delineating the capacity of canine blood cells to mount a humoral immune response.

Filter spot-ELISA for the enumeration of interferon-alpha antibody-secreting cells.

A filter spot-ELISA was developed for the enumeration of interferon-alpha (IFN-alpha) antibody secretion by murine and human cells. Various human IFN-alpha subtypes were coated onto nitrocellulose membranes by means of broad specificity bovine antibodies. Membranes were blocked with milk proteins, and antibodies released by individual cells during a 3 h culture period were visualized as distinct spots using peroxidase-conjugated, species-specific anti-immunoglobulin antibodies and diaminobenzidine/hydrogen peroxide substrate solution. The spot-ELISA is rapid, easy to perform, and highly sensitive. Possible applications include frequency estimates of IFN-alpha antibody-secreting cells in blood, tissues and hybridoma cultures as well as the evaluation of specificity and immunoglobulin class of the respective antibodies.

Fetal liver transplantation in the dog. I. Restoration of hemopoiesis with cryopreserved fetal liver cells from DLA-identical siblings.

Fetal liver cells (FLC) were obtained from beagle fetuses 52 days postconception, and were cryopreserved prior to transplantation into ten sibling recipients that had previously been exposed to total-body irradiation delivered in 3 fractions of 6 Gy each at 4 days, 2 days, and 2 hr before grafting. Donors and hosts were genotypically identical for dog leukocyte antigens (DLA)-A, B, and D. A rapid and lasting engraftment was achieved in all animals following the transfer of 0.2 X 10(8) to 1.6 X 10(8) mononuclear FLC/kg body weight, which were equivalent to 0.9 X 10(4) to 19.8 X 10(4) granulocyte/macrophage progenitor cells (CFU-GM)/kg. Between days 14 and 20 posttransplant pretreatment levels were detected for blood granulocytes, between days 23 and 28 for circulating platelets, and between days 35 and 40 for the erythrocyte count and hemoglobin concentration. Increasing the number of CFU-GM transfused resulted in an accelerated granulocyte and platelet recovery. Bone marrow cells were of donor origin throughout the observation interval, but declining proportions of host lymphocytes circulated in the peripheral blood during the initial recovery phase. In two dogs, skin alterations that might indicate slight graft-versus-host disease (GVHD) were noted following days 20 and 70, respectively. Six recipients had to be sacrificed due to inanition, probably secondary to radiation-induced pancreatic insufficiency two to three months after grafting. The results of this study indicate that cryopreserved FLC are highly effective in restoring hemopoiesis in DLA-compatible sibling dogs. Transplantation of canine FLC may prove valuable in analyzing mechanisms pathogenetically related to graft rejection or to the development of GVHD following the transfer of T-cell-depleted hemopoietic grafts at a preclinical stage.

Fetal liver transplantation in the dog. II. Repopulation of the granulocyte-macrophage progenitor cell compartment by fetal liver cells from DLA-identical siblings.

The restoration of the granulocyte-macrophage progenitor cell (CFU-GM) compartments in blood and bone marrow, and the recovery of blood monocytes were followed for up to one year in ten beagles that had been exposed to fractionated (3 X 6 Gy) total-body irradiation before being transfused with cryopreserved fetal liver cells (FLC) from sibling donors that were genotypically matched for dog leukocyte antigens. Grafts contained 0.2-1.6 X 10(8) mononuclear cells and 0.9-19.8 X 10(4) CFU-GM/kg body weight. Numbers of circulating monocytes rose parallel to granulocyte numbers after day 6 and became normal by day 18 posttransplant. In bone marrow aspirates, low numbers of CFU-GM were detected on day 3 and their incidence per 10(5) mononuclear cells was normal after day 14. Circulating CFU-GM were present in significant numbers by day 7 and their elevated concentration per milliliter of blood after day 14 continued for one year. Dextran sulfate injection mobilized normal numbers of CFU-GM into the blood early after transplantation, and spontaneously circulating CFU-GM in a later phase did not differ from blood progenitors of normal animals with respect to radiation sensitivity and sedimentation velocity. Thus, FLC transplantation effected a rapid restoration of granulopoiesis and monocytopoiesis, which was reflected at both the level of mature blood cells and the compartments of CFU-GM in blood and bone marrow, underlining the high repopulating capacity of fetal liver stem cells.

Development and histogenesis of the thymus in dog. A light and electron microscopical study.

The development of the thymus was studied with histological, transmission electron microscopic (TEM) and histochemical methods in 100 dog fetuses (beagle), between day 19 of gestation and day 21 after birth. Thymus development could be divided in three stages: 1/Formation of epithelial palisades; 2/Initiation of lymphopoiesis; 3/Differentiation of the medulla and Hassall's bodies (HB). The epithelial anlagen were seen at day 23 of gestation showing the characteristic palisade structure of the endodermally derived epithelium. Ten days later the beginning of lymphoiesis and the reticularization of the epithelial cells could be seen. The first HB could be found at day 38 when cortical-medullary differentiation is recognized. The histochemical observation demonstrated a rich content of PAS positive coarse granules in the cytoplasm of reticulo-epithelial (RE) cells. On the other hand, the HB showed a diffuse PAS positive reaction. The ultrastructural investigations demonstrated the presence of desmosomes connecting RE cells to one another. Desmosomes were not found between RE and lymphocytes. The growth of the developing thymus into the mesenchymal matrix resulted in the lobulation of the organ by connective tissue cells and fibers. The first mast cells were seen at day 35 of gestation, most abundantly in the interlobular connective tissue (ICT) although a few were present in the cortex and somewhat more in the medulla, near the HB. At the end of development small groups of neutrophil cell precursors appeared in the ICT and the cortex. Cysts were not present up to day 21 after birth.

High-titre interferon-alpha antibodies in a patient with chronic graft-versus-host disease after allogeneic bone marrow transplantation.

In a patient undergoing allogeneic BMT for chronic phase CML, de novo chronic GVHD developed within 80 days after transplantation. Eighteen months post-BMT, high serum levels of neutralizing interferon-alpha (IFN-alpha) antibodies were detected, which persisted despite continuous immunosuppressive treatment. The antibodies were of oligoclonal or polyclonal origin, predominantly of the IgG1 type, and reacted broadly with various human IFN-alpha types, including the patients endogenous IFN-alpha, but failed to recognize natural IFN-beta and recombinant IFN-gamma. Pathogenesis and clinical impact of the IFN-alpha antibodies are unknown. Antibodies of cytokines are a novel class of autoantibodies that may develop after allogeneic BMT and interfere with cytokine homeostasis and immune regulation.

Antibodies to interferon-alpha: a novel type of autoantibody occurring after allogeneic bone marrow transplantation.

Antibodies to IFN-alpha have been recognized as a novel type of autoantibody developing after allogeneic BMT. Ninety-six patients undergoing BMT for various hematologic disorders were followed for the presence of spontaneous IFN-alpha antibodies until 12 years after transplantation. Seven of them (7.3%) developed IFN-alpha antibodies occurred late after BMT (> or = 15 months), rose to very high titers in some patients, and persisted for years despite combined immunosuppressive treatment. They were oligo- or polyclonal in nature, predominantly IgG with a broad IgG subclass distribution, and neutralized the antiviral and antiproliferative activity of various natural and recombinant IFN-alpha types including the patients' endogenous IFN-alpha in vitro. All antibody-positive recipients suffered from chronic GVHD (n = 5) or chronic viral hepatitis (n = 2), but the only significant association was with prior severe aplastic anemia (3/9, 33%; P = 0.022). There was no discernible HLA association of IFN antibody development. Although the clinical relevance of the IFN-alpha antibodies is uncertain they may interfere with cellular defence mechanisms and immune regulation after BMT.

Bone marrow structure and its possible significance for hematopoietic cell renewal.

The authors review the progress made during the last quarter of a century in the fields of hematopoietic cellular proliferation and differentiation in relation to the bone marrow structure and the microenvironment provided by the marrow stroma in which unlimited self-renewal occurs. The marrow is conceived of as an organ in which the stroma originates from local mesenchymal elements which form a vascularized and innervated matrix, seeded later by blood-borne stem cells. Transplantation studies using total-body-irradiated dogs show that stem cells derived from the marrow, as well as those from the blood and from the fetal liver, are able to repopulate a marrow rendered aplastic by irradiation. By grafting equal numbers of GM-CFU from peripheral blood and bone marrow, a faster hemopoietic reconstitution is provided by blood-derived stem cells. The most efficient stem cells in the long range are those derived from fetal liver. Bone marrow and peripheral blood GM-CFU differ in some in vitro characteristics such as radiation sensitivity. These peripheral blood cells are more radiosensitive than those derived from the marrow. Autografting of bone marrow mononuclear cell fractions obtained by velocity sedimentation techniques demonstrates that the fraction of small mononuclear cells holds a repopulating potential similar to that of circulating blood stem cells. The cells collected in fraction 2 of a discontinuous albumin gradient contain most of the blood stem cells and repopulate the marrow without causing GVHD, while cells collected in fractions 3 and 4 contain a minimal amount of stem cells and cause severe GVHD.

Photodynamic therapy in psoriasis: suppression of cytokine production in vitro and recording of fluorescence modification during treatment in vivo.

Photodynamic therapy (PDT) consists of the combination of photosensitizers absorbing light mainly in the red spectral region and irradiation with light of corresponding wavelengths. We analysed its effects on the cytokine secretion (IL-1 beta, TNF alpha, IL-6) of freshly isolated peripheral mononuclear cells from six patients with chronic plaque-stage psoriasis in comparison with PUVA. PUVA treatment resulted in a decreased production of all three cytokines, but most pronounced in the case of IL-6. PDT caused a similar change in the cytokine pattern, but its effectiveness was lower. In vivo fluorescence recordings were performed on psoriatic plaque lesions after topical application of the photosensitizer Photosan-3. Under irradiation, progressive photobleaching was noted with increasing radiation dosage. This is the first reported study of photochemical reactions using on-line fluorescence recordings during PDT of psoriatic lesions in vivo. Our results demonstrate the capacity of PDT to cause immunomodulatory effects similar to PUVA, thus indicating its potential application to the treatment of this common disease.

Incidence and in-vivo relevance of anti-interferon antibodies during treatment of low-grade cutaneous T-cell lymphomas with interferon alpha-2a combined with acitretin or PUVA.

Interferon-alpha combined with retinoid or PUVA is used for the treatment of cutaneous T-cell lymphoma. Anti-IFN-alpha antibodies (IFN ab) occur regularly during IFN-alpha treatment. We investigated the incidence of neutralizing and binding IFN ab and analysed their relationship with clinical and immunological parameters. A group of 17 CTCL patients were treated with IFN alpha-2a three times weekly subcutaneously at a dose of 3 Mill. I.U. combined either with retinoid (acitretin, Neotigason; 0.5 mg/kg bodyweight) daily or with 5-methoxypsoralen (1.2 mg/kg bodyweight) plus UVA radiation three times weekly. Prior to and during treatment we monitored stage, skin involvement by a tumour burden index, serum levels of beta 2-microglobulin, neopterin, binding and neutralizing IFN ab, Interleukin-6 (IL-6), soluble IL-2 receptors (sIL-2r) and the CD4/CD8 ratio of peripheral blood mononuclear cells. We observed two complete, two partial and six minor responses, four patients with stable disease and three patients with progressive disease. Of the 17 patients, 7 developed binding IFN ab, but only 2 had neutralizing IFN ab which were associated with high titres of binding IFN ab. IFN ab formation was more frequent in patients with normal CD4/CD8 ratios and a high tumour burden index and showed a trend to be more frequent in PUVA-cotreated patients than in retinoid-cotreated patients. Responses were more frequently seen in IFN ab-negative patients. IFN ab developed in patients treated with PUVA or retinoid combined with IFN. Binding as well as neutralizing IFN ab may have an impact on the treatment success in CTCL patients.

Interferon-alpha antibodies in patients with renal cell carcinoma treated with recombinant interferon-alpha-2A in an adjuvant multicenter trial. The Delta-P Study Group.

BACKGROUND: Prolonged therapy with interferon (IFN) may lead to the formation of IFN antibodies. METHODS: Patients with renal cell carcinoma (n = 270) with advanced localized disease were randomized after complete tumor resection to receive treatment with adjuvant recombinant IFN-alpha-2a (rIFN-alpha 2a) (9 x 10(6) IU subcutaneously, three times per week for a maximum of 12 months) versus no treatment. Patients (IFN-treated group, 106 patients; control group, 97 patients) were monitored for the presence of rIFN-alpha 2a antibodies. RESULTS: Of 86 IFN-treated patients observed for more than 2 months, 40 (47%) had IFN-alpha 2a-binding and 25 (29%) had IFN-alpha 2a-neutralizing antibodies developed within a median of 3 and 6 months, respectively. A distinct peak in binding antibody titers occurred at 6-9 months. Therapy-induced neutralizing antibodies were equally reactive with two other recombinant IFN-alpha-2 subtypes but poorly recognized natural IFN-alpha (IFN-alpha), recombinant IFN-alpha-1/alpha-8, and recombinant IFN-omega-1. The duration of remission and rate of relapse were independent of the antibody status, although neutralizing and most non-neutralizing antibodies correlated with a reduction in the IFN-induced increase in beta-2-microglobulin levels. CONCLUSIONS: Patients treated with IFN-alpha 2a should be monitored for the presence and clinical relevance of IFN-alpha antibodies to determine those who could respond to alternative treatment.

Treatment-induced antibodies to interleukin-2.

Interleukin-2 (IL-2) is a 15 kDa glycoprotein with proven activity as an immune stimulant in the treatment of malignant disorders, congenital and acquired immune deficiencies, infectious disorders, and as an adjuvant to vaccines. Both natural and recombinant type IL-2 preparations have been applied in clinical treatment trials and have turned out to be immunogenic, although to a varying extent. Enzyme immunoassays and western blotting are standard procedures for the detection of IL-2-binding antibodies, whereas the neutralizing capacity of these antibodies is frequently demonstrated by inhibition of IL-2-dependent cell growth in vitro. The rate of treatment-induced IL-2 antibodies has varied from 0% to 100% in reported trials and frequently exceeded 50% in patients exposed to recombinant IL-2, whereas natural type IL-2 appeared to be little immunogenic. Duration of treatment, cumulative IL-2 dose, and route of IL-2 administration are likely to determine both the rate of seroconversion as well as composition and properties of the anti-IL-2 antibodies. Interleukin-2 antibodies are polyclonal in nature and predominantly composed of IgM and IgG types. Frequently they react with both recombinant and natural IL-2 types. As a rule, neutralizing IL-2 antibodies are detected in serum samples with high IL-2-binding titers and are recognized later than their non-neutralizing predecessors. Neutralization in vitro, however, does not predict neutralization in vivo, and there are very rare patients with documented, antibody-mediated loss of response to IL-2 treatment. More frequently, IL-2 antibodies will limit the expression of IL-2-dependent proteins in vivo, but the opposite has also been observed. Although the precise mechanism of antibody induction by IL-2 is unknown, immunogenicity of some drug formulations rather than polyclonal B-cell activation appears to play a critical role. Approaches aiming at limiting the immunogenicity of IL-2 preparations are discussed, and strategies how to recognize and circumvent antibody-mediated IL-2 resistance are presented.