Treatment of rheumatoid arthritis with a recombinant human tumor necrosis factor receptor (p75)-Fc fusion protein.

BACKGROUND: Tumor necrosis factor (TNF) is a proinflammatory cytokine involved in the pathogenesis of rheumatoid arthritis, and antagonism of TNF may reduce the activity of the disease. This study evaluated the safety and efficacy of a novel TNF antagonist - a recombinant fusion protein that consists of the soluble TNF receptor (p75) linked to the Fc portion of human IgG1 (TNFR:Fc). METHODS: In this multicenter, double-blind trial, we randomly assigned 180 patients with refractory rheumatoid arthritis to receive subcutaneous injections of placebo or one of three doses of TNFR:Fc (0.25, 2, or 16 mg per square meter of body-surface area) twice weekly for three months. The clinical response was measured by changes in composite symptoms of arthritis defined according to American College of Rheumatology criteria. RESULTS: Treatment with TNFR:Fc led to significant reductions in disease activity, and the therapeutic effects of TNFR:Fc were dose-related. At three months, 75 percent of the patients in the group assigned to 16 mg of TNFR:Fc per square meter had improvement of 20 percent or more in symptoms, as compared with 14 percent in the placebo group (P<0.001). In the group assigned to 16 mg per square meter, the mean percent reduction in the number of tender or swollen joints at three months was 61 percent, as compared with 25 percent in the placebo group (P<0.001). The most common adverse events were mild injection-site reactions and mild upper respiratory tract symptoms. There were no dose-limiting toxic effects, and no antibodies to TNFR:Fc were detected in serum samples. CONCLUSIONS: In this three-month trial TNFR:Fc was safe, well tolerated, and associated with improvement in the inflammatory symptoms of rheumatoid arthritis.

Interleukin-1 alpha-mediated promotion of long-term alloengraftment and short-term neutrophil expansion does not require the presence of either donor or host T cells.

In earlier studies, we showed that a 14-day continuous subcutaneous infusion of recombinant human interleukin (IL)-1 accelerated neutrophil recovery and enhanced long-term chimerism in a bone marrow (BM) transplant model in which T-cell-depleted BALB/c donor BM was given to irradiated C57BL/6 fully allogeneic recipients. We have extended these studies to a model entirely devoid of donor and host T cells. In the model, donor BALB/c congenic severe combined immunodeficient (C.B-17-scid/scid) BM cells are T cell depleted. The cells are then transplanted into adult irradiated C57BL/6 hosts that have been thymectomized and treated with anti-CD4 and CD8. When IL-1 alpha was delivered subcutaneously using a mini-osmotic pump, it enhanced short-term neutrophil recovery and longer term alloengraftment despite the absence of T cells in the donors and the hosts. Therefore, T cells were not required for the promotional effects of IL-1 alpha on neutrophil recovery and alloengraftment. Studies also showed that the potency of the IL-1 alpha effects was related to the degree of donor cell engraftment, which was related to the irradiation dose and the presence of T cells. We conclude that IL-1 alpha can augment post-BM transplantation hematopoietic recovery and alloengraftment via a T-cell-independent mechanism by favoring donor allogeneic hematopoietic progenitor cell competition over limited numbers of host progenitor cells.

IL-15 administration following syngeneic bone marrow transplantation prolongs survival of lymphoma bearing mice.

The toxicity and antitumor efficacy of simian IL-15 was compared with human IL-2 in the context of syngeneic BMT. Groups of mice receiving or not receiving anti-CD3 activated splenocytes, termed "T-activated killer" (T-AK) cells, were treated between days 7 and 12 with escalating doses of IL-2 or IL-15 given twice daily. Recipients of IL-2+T-AK or IL-15+T-AK had significantly higher survival rates than saline+T-AK. Tissues from IL-2+T-AK, but not IL-15+Y-AK, treated mice revealed the presence of perivascular infiltrates in the lung and liver consisting of CD8+ T cells and Mac-1+ cells. Our findings demonstrate that IL-15 can be used effectively to stimulate antitumor responses post-BMT and may be associated with less toxicity than IL-2.

Interaction of IL-15 with the shared IL-2 receptor beta and gamma c subunits. The IL-15/beta/gamma c receptor-ligand complex is less stable than the IL-2/beta/gamma c receptor-ligand complex.

This study was designed to compare the interactions of IL-2 and IL-15 with the IL-2R beta and IL-2R gamma c subunits, as differences in receptor interactions between IL-2 and IL-15 might contribute to the functional differences between these two cytokines. The results suggest the existence of a human IL-15R alpha subunit, although physical evidence of this molecule was not obtained. Proliferation of anti-CD3 (OKT3)-stimulated human PBL was compared for responsiveness to IL-2, IL-15, and F42K, and IL-2 mutant that does not bind the IL-2R alpha chain. F42K was more potent than IL-15 in activating a dose-dependent response. This fact, along with Scatchard binding analyses of IL-15 on OKT3 blasts and YT cells revealing both high and intermediate affinity receptors, supports the existence of IL-15R alpha on these cells. Additional characterization of the IL-15R utilized covalent cross-linking to affinity label IL-2R and IL-15R on YT cells and OKT3 blasts. Consistent with previously reported functional data, IL-2R alpha was not co-precipitated from the [125I]IL-15 receptor-ligand complex, demonstrating that IL-15 does not interact physically with the IL-2R alpha subunit. While IL-2R alpha did co-precipitate with IL-2R beta and IL-2R gamma c in the presence of IL-2, IL-15R alpha did not co-precipitate with the IL-2R beta/gamma c complex. Finally, YT cells equilibrated with IL-2 and then precipitated through IL-2R beta showed that IL-2R beta and IL-2R gamma c co-precipitate in a 1:1 ratio, while only IL-2R beta was found in the immunoprecipitates of YT cells equilibrated with IL-15. This indicates that IL-15 creates a less stable bridge between the IL-2R beta and IL-2R gamma c chains than does IL-2 on YT cells. This result was identical for both surface-iodinated YT cells and immunoprecipitates that were probed for IL-2R beta and IL-2R gamma c on Western blots.

Effect of IL-7 or IL-4 on reconstitution of donor lymphoid cells in congenic murine bone marrow transplantation.

IL-7 and IL-4 are known to influence the growth of cells of the lymphoid lineage. In this study, we investigated the effects of in vivo administration of IL-7 or IL-4 in mice subjected to congenic BM transplant. C57BL/6 Ly5.1+ mice were subjected to TBI, followed by transfer of B and T cell-depleted BM from C57BL/6 Ly5.2+ donor mice. Recipient mice were implanted with 14-day miniosmotic pumps that delivered IL-7, IL-4 or PBS and were examined for reconstitution of lymphoid cells using flow cytometry on different days. We observed no significant difference in the number of splenocytes, thymocytes and PBLs between recipient mice administered with cytokines or normal control mice. However, we observed that IL-4 infusion resulted in appearance of increased numbers of donor CD23+B220+ cells and also donor cells expressing Fc receptors for IgM (Fc micro R) and B220. Since CD23 is present only on mature B cells, our data demonstrate that following BMT, IL-4 treatment results in the development of more mature B cells compared to control mice. Additionally, we observed that IL-7 infusion resulted in significantly decreased expression of donor sIgM+B220+ cells. However, the effects of IL-7 or IL-4 were observed when the cytokines were actively administered and rapidly abated upon cessation of cytokine therapy.

Regulation of endothelial VCAM-1 expression in murine cardiac grafts. Expression of allograft endothelial VCAM-1 can be manipulated with antagonist of IFN-alpha or IL-4 and is not required for allograft rejection.

This report provides evidence to support the hypothesis that tumor necrosis factor-alpha (TNF-alpha) and IL-4 promote the expression of new endothelial surface molecules in rejecting murine heterotopic cardiac allografts. The microvascular endothelia of these cardiac allografts all develop strong reactivity with the monoclonal antibodies (mAbs) YN1.1/74 (anti-ICAM-1), M/K-2 (anti-VCAM-1) and MECA-32 (undefined molecule) within 3 to 5 days of graft implantation. Daily treatment of the allograft recipients with pentoxifylline (PTX), soluble TNF receptor (TNFR:Fc), anti-interleukin-4 (IL-4) mAb (11B11), or soluble IL-4 receptor, each abrogate the expression of endothelial VCAM-1 and reduce the endothelial reactivity with the mAbs YN1.1/74 and MECA-32 to levels found in cardiac isografts. This is accompanied by a reduction, but not an elimination, of interstitial leukocytic infiltration. Despite this, cardiac allograft recipients treated with PTX or the mAb 11B11 rejected allografts at the same rate as untreated allograft recipients, ie, within 10 to 12 days after graft implantation. These rejected grafts contained mRNAs for TNF-alpha and IL-4, as well as for all other cytokines that have been associated with rejecting allografts. This suggests that endothelial VCAM-1 expression, which is characteristic of rejection, is not an essential element of the rejection process. Interestingly, the grafts from the PTX-treated recipients continued to display rare, isolated VCAM-1 positive cells in the interstitium, which may be dendritic cells. In general, these studies demonstrate a role for IL-4 and TNF-alpha in the alterations of vascular endothelial phenotype observed in rejecting cardiac allografts. They also demonstrate that endothelial VCAM-1 expression is not essential for the allograft rejection process, and that the role of VCAM-1 in this process may be more subtle than was initially suspected.

Inhibition of the effects of TNF in renal allograft recipients using recombinant human dimeric tumor necrosis factor receptors.

Tumor necrosis factor alpha (TNFa) and lymphotoxin (LT) or TNF beta are closely linked cytokines produced by macrophages and activated T lymphocytes, which play important regulatory roles in the immune response to allografts. They have also been implicated as mediators of the adverse reactions observed during OKT3 therapy. Therefore, anti-TNF agents could be useful both for immunosuppression and for limiting the systemic response observed in patients receiving OKT3. Recombinant TNFR:Fc is a fusion protein that binds TNFa and LT, thereby neutralizing their effects in vitro. The present study investigates the potential clinical application of TNFR:Fc in a nonhuman primate renal allograft model. Cynomolgus renal allograft recipients were treated with TNFR:Fc induction therapy alone or in combination with subtherapeutic doses of cyclosporine. Control animals received no immunosuppression or subtherapeutic cyclosporine. TNFR:Fc, administered as the only immunosuppressive agent, successfully prolonged renal allograft survival in the majority of treated animals. The prolongation of allograft survival was even more impressive when TNFR:Fc was combined with subtherapeutic doses of cyclosporine. Onset of rejection was significantly delayed as well in the TNFR:Fc treated groups. No adverse side effects were observed in any of the TNFR:Fc treated animals. Precursor cytotoxic T cells were detected in peripheral blood samples of treated recipients but the level of effector CTLs in vivo was below the threshold of detection. These results demonstrate that TNFR:Fc can be safely administered and is effective in prolonging renal allograft survival and in delaying the onset of rejection when administered alone or in combination with cyclosporine.

A role for IL-4 in immunologically mediated enteropathy.

A number of clinical enteropathies are associated with a local cell-mediated immune (CMI) response, and experimental evidence indicates that cytokines are responsible for the intestinal pathology. We show here that depletion of IL-4 using MoAb or a soluble form of the IL-4 receptor (IL-4R) prevents the jejunal manifestations of a proliferative form of murine graft-versus-host reaction (GVHR) characterized by crypt hyperplasia and recruitment of intraepithelial lymphocytes (IEL). Depletion of IL-4 did not prevent the appearance of villus atrophy in a destructive model of GVHR, and had no effect on any indices of systemic immunity. These results indicate that IL-4 may play a selective role in mediating proliferative aspects of intestinal immunopathology, and suggest that this cytokine may provide a useful target for immunotherapy.

Steel factor (c-kit ligand) stimulates the in vitro growth of immature CD3-/CD4-/CD8- thymocytes: synergy with IL-7.

The ability of Steel factor (SLF) to stimulate the growth of immature thymocytes alone and in combination with IL-7 was assessed. In suspension cultures of CD4-/CD8- thymocytes, SLF itself did not induce significant proliferation, but in combination with IL-7, it induced a greater response than did IL-7 alone. In fetal thymus lobe submersion cultures, SLF stimulated the growth of cells to a magnitude similar to that of IL-7 and the combination resulted in a synergistic response. Phenotypic analysis of these cells revealed that SLF in comparison to IL-7 stimulated the growth of a less mature population. The synergistic growth stimulated by the combination of IL-7 and SLF occurred in a population of IL-2R+/CD4-/CD8-/CD3- cells. Cells grown in SLF expressed low levels of IL-2R and overnight culture in IL-7 dramatically increased IL-2R expression. Thus, these results are consistent with the hypothesis that SLF stimulates the growth of a less mature thymocyte population than does IL-7.

Helper T cell-independent proliferation of CD8+ cytotoxic T lymphocytes transduced with an IL-1 receptor retrovirus.

Although the proliferation of CD8+ CTL typically requires cytokine support provided by helper T cells, a subset of naturally occurring CD8+ CTL are capable of proliferating independently of T cell help. Such helper-independent CTL have previously been shown to possess IL-1 receptors (IL-1R) and to proliferate in response to IL-1 through endogenous production of IL-2. In this study, we have transduced conventional helper-dependent CTL clones with a retroviral vector encoding the murine type I IL-1R. Transduced CTL selected in G418 expressed vector-derived transcripts encoding IL-1R and displayed approximately 1000 cell surface receptors with an IL-1 affinity typical for the type I IL-1R. In contrast to parental cells, transduced CTL proliferated in response to IL-1 in the presence of Ag, without a requirement for helper T cells, IL-2, or other cytokine support. Stimulation with both IL-1 and Ag was necessary for the proliferative response. No endogenous synthesis of IL-2 could be detected in the IL-1R transduced cells in response to IL-1 stimulation, in the presence or absence of Ag. The IL-1R-induced phenotype was demonstrated in two independent T cell clones, both of which retained Ag-specific cytolytic activity. No such conversion to a helper-independent phenotype was induced by a retroviral vector encoding only the neo gene. The behavior of the IL-1R-transduced CTL in proliferation assays thus resembled that of the naturally occurring helper-independent CTL.

Effect of total body irradiation, busulfan-cyclophosphamide, or cyclophosphamide conditioning on inflammatory cytokine release and development of acute and chronic graft-versus-host disease in H-2-incompatible transplanted SCID mice.

In a previous study, we found that total body irradiation (TBI) was essential to induce acute graft-versus-host disease (GVHD) after allogeneic H-2-incompatible splenocyte (SP) transplantation in SCID mice. SCID mice (H-2d) conditioned with cyclophosphamide and transplanted intravenously (IV) with 5 x 10(7) C57BL/6 (H-2b) SP developed chronic GVHD within 3 months posttransplant without any evidence of preceding acute GVHD. In this study, SCID mice were conditioned with 4 Gy TBI or non-TBI regimens, either BuCy2 (busulfan 4 mg/kg/d + cyclophosphamide 100 mg/kg/d for 2 days) or Cy5 (cyclophosphamide 100 mg/kg/d for 5 days), and then transplanted IV with 5 x 10(7) SP. The TBI-conditioned mice were further divided into tree transplant groups: (1) TBI and SP administered the same day (TBI + D0 SP), (2) SP administered 4 days post-TBI (TBI + D4 SP), and (3) SP administered 7 days post-TBI (TBI + D7 SP). The severity of GVHD was compared among these groups by clinical and histologic grading. Twenty-eight of 28 mice treated with TBI + D0 SP died of acute GVHD, with overwhelming diarrhea by day 15 posttransplantation. Sixteen mice treated with either TBI + D4 SP or TBI + D7 SP developed acute GVHD, but none of them died of this disorder during 30 days posttransplantation. The mice conditioned with non-TBI regimens developed chronic GVHD within 3 months without showing any detectable signs of acute GVHD. Serum and in situ colonic cytokines were determined by enzyme-linked immunosorbent assay and immunohistology respectively. TBI itself significantly increased both serum and colonic tumor necrosis factor-alpha (TNF-alpha), interleukin-1 alpha (IL-1 alpha), and IL-6 when compared with non-TBI regimens and normal controls. TNF-alpha appeared in the serum and colon 4 hours post-TBI and peaked in 24 hours, followed by increasing IL-1 alpha and then IL-6 levels. TNF-alpha and IL-1 alpha decreased rapidly within 3 to 5 days post-TBI if no allogeneic cells were transplanted. Histoincompatible transplantation augmented cytokine release, which remained elevated on day 10 in these animals. Mice treated with TBI + D0 SP developed the most severe acute GVHD and had the highest levels of TNF-alpha, IL-1 alpha, and IL-6. The BuCy2-conditioned mice had the lowest cytokine levels and developed no acute GVHD. When the mice transplanted with TBI + D0 SP were treated immediately with recombinant soluble human TNF receptor (rhuTNFR:Fc) 100 micrograms/d intraperitoneally and for the subsequent 15 days acute GVHD mortality was significantly reduced from 100% to 50% (P < .001).(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of a recombinant human soluble tumor necrosis factor receptor FC fusion protein on type II collagen-induced arthritis in mice.

A recombinant human TNF receptor Fc fusion protein (rhuTNFR:Fc) was assessed for antiarthritic activity using murine type II collagen-induced arthritis in mice. DBA/1 mice were immunized with bovine type II collagen and treated with rhuTNFR:Fc either from day 21 to day 28 (preventative protocol), or after disease onset for fourteen days (therapeutic protocol). Control mice received either sterile saline or human serum albumin injections. rhuT-NFR:Fc treatment significantly reduced both the incidence and the severity of collagen-induced arthritis in the preventative protocol. Mice receiving rhuTNFR:Fc therapeutically progressed to less severe disease than did control animals, and the arthritis index in rhuTNFR:Fc treated mice was significantly lower than the index in control mice from 7.5 weeks after treatment. The antibody response to collagen was significantly reduced by treatment with rhuTNFR:Fc in both the preventative and therapeutic protocols. No difference was observed in the proliferative response to type II collagen or Con A, but the response to LPS was significantly lower in rhuTNFR:Fc treated mice at the conclusion of both the preventative and therapeutic trials. The results suggest that rhuTNFR:Fc may have both immunosuppressive and antiarthritic properties in this experimental model, and may represent a useful approach to the treatment of autoimmune arthritis.

Soluble tumor necrosis factor (TNF) receptors are effective therapeutic agents in lethal endotoxemia and function simultaneously as both TNF carriers and TNF antagonists.

Two forms (monomeric or dimeric) of the extracellular, ligand-binding portion of the human p80 cell-surface receptor for TNF were used to antagonize TNF activity in vitro and in vivo. The dimeric sTNFR:Fc molecule was a more potent inhibitor of TNF than the monomeric sTNFR (50 to 1000x), as assessed in vitro by inhibition of TNF binding or bioactivity and in vivo by protection of mice from an otherwise lethal injection of LPS. Surprisingly, the dimeric sTNFR:Fc construct demonstrated a beneficial effect even when administered 3 h after a lethal LPS injection (i.e., after serum TNF levels had peaked and receded). To study the mechanism by which the soluble TNFR functions in vivo, serum TNF levels were examined in mice given LPS in the presence or absence of soluble receptor. Administration of a mortality-reducing dose of sTNFR:Fc ablated the rise in serum TNF bioactivity that normally occurs in response to LPS. However, TNF bioactivity was revealed in these "TNF-negative" serum samples when the L929 bioassay was modified by inclusion of a mAb that blocks the binding of murine TNF to the human soluble TNFReceptor. These results indicate that the absence of direct cytolytic activity in the L929 assay was caused by neutralization of TNF, rather than to an absence of TNF in the serum. Moreover, administration of either monomeric sTNFR or low doses of dimeric sTNFR:Fc actually resulted in increased serum TNF levels compared to mice given LPS but no soluble receptor. However, these "agonistic" doses of soluble receptor did not lead to increased mortality when an LD60 dose of LPS was given. Thus, dimeric sTNFR are effective inhibitors of TNF and under some circumstances function simultaneously as both TNF "carriers" and antagonists of TNF biologic activity.

Recombinant soluble murine IL-4 receptor can inhibit or enhance IgE responses in vivo.

This study examines the effects of soluble IL-4R (sIL-4R) administration on IgE production in vivo by using an anti-IgD injection model. Anti-IgD-treated mice were given various doses of sIL-4R or anti-IL-4 mAb over a 3-day period and serum IgE levels were determined by ELISA on day 9. The sIL-4R inhibited IgE production by up to 85%. Anti-IL-4 mAb administration resulted in comparable levels of inhibition at considerably lower doses. The disparity in efficacy between sIL-4R and anti-IL-4 mAb was likely the result of differences in the biodistribution and in vivo half-life of the two IL-4-binding proteins. The specificity of the sIL-4R inhibitory effect was assessed by mixing sIL-4R with various concentrations of IL-4 before injection. Exogenous IL-4 partially overcame the inhibitory effect of high-dose sIL-4R or anti-IL-4 mAb. Unexpectedly, coadministration of suboptimal concentrations of anti-IL-4 mAb or sIL-4R with IL-4 resulted in superinduction of the IgE response. This stimulatory effect was dose dependent for both IL-4 and the IL-4 cognates and was not seen in the absence of exogenous IL-4 over the entire concentration range tested for either sIL-4R or anti-IL mAb. The results indicate that sIL-4R can block IgE secretion by neutralizing endogenous IL-4. Furthermore, sIL-4R can enhance, in a dose-dependent manner, the biologic effects of exogenously administered IL-4, presumably by altering the biodistribution of the cytokine. These findings suggest two alternative applications for cytokine-binding proteins, i.e., 1) as antagonists of biologic activities of endogenously produced cytokines and, 2) as vehicles for cytokine delivery.

Promotion of murine marrow alloengraftment and hematopoietic recovery across the major histocompatibility barrier by administration of recombinant human interleukin-1 alpha.

Irradiated C57BL/6 (H-2b) recipients of T-cell-depleted (TCD) BALB/c (H-2d) bone marrow (BM) and recombinant interleukin-1 alpha (IL-1 alpha) (1 microgram/d) had a significantly (P less than or equal to .006) higher 100-day actuarial survival rate, accelerated hematopoietic recovery, and higher levels of alloengraftment than a group of transplanted control mice treated identically, but given phosphate-buffered saline (PBS). To elucidate the mechanisms involved with IL-1 alpha-induced promotion of alloengraftment and hematopoietic recovery, we performed sequential splenic FACS studies on transplanted mice and secondary transfer studies in syngeneic mice given IL-1 alpha or PBS. Splenic phenotyping showed that recipients of IL-1 alpha had a higher proportion of donor granulocytes (52% v 19%) as compared with PBS controls as early as 7 days after bone marrow transplantation (BMT). On day 11 post-BMT, recipients of IL-1 alpha had a more than fourfold increase in splenocyte number, which included a higher percentage (90% v 59%) of donor cells, especially donor granulocytes (52% vs 32%), and a sevenfold increase in donor T cells as compared with controls. Host T-cell numbers were not affected. Taken together, these data suggest that IL-1 alpha stimulated bipotential (myeloid and lymphoid) donor cell engraftment. In a syngeneic BMT system, administration of IL-1 alpha resulted in a higher incidence of survival when recipients were transplanted with BM cells, indicating that IL-1 alpha administration probably either expanded or potentiated engraftment of a committed progenitor cell pool. Secondary transfer experiments using marrow from IL-1 alpha-treated mice showed that the number of day 12 colony-forming unit-spleen (CFU-S) cells was unaltered compared with untreated control mice, suggesting that more primitive, albeit committed, hematopoietic progenitor cells were not affected. We also examined the potential additive effects of IL-1 alpha and granulocyte-macrophage colony-stimulating factor (GM-CSF) administered in combination (for 14 days). Mice receiving a suboptimal amount of IL-1 alpha along with GM-CSF had significantly higher levels of donor alloengraftment (92%) with superior hematopoietic recovery, as compared with mice receiving either IL-1 alpha (57%) or GM-CSF (18%) alone.

Beta 2-microglobulin-, CD8+ T-cell-deficient mice survive inoculation with high doses of vaccinia virus and exhibit altered IgG responses.

Transgenic mice lacking an intact beta 2-microglobulin (beta 2m) gene fail to express major histocompatibility complex (MHC) class I proteins on the cell surface and, as a result, are virtually devoid of CD4- CD8+ lymphocytes. These animals provide a unique model system for directly assessing the role of CD8+ lymphocytes in the modulation of viral infection in vivo. beta 2m- CD8- mice and their normal littermates were inoculated at the base of the tail with the WR strain of vaccinia virus and monitored for serum antibody and lesion formation. Both groups developed similar lesions in response to a broad virus dose range, and all animals had completely recovered by day 28 after inoculation. Isotype-specific immunoglobulin levels were determined for each animal on day 7 and day 14 after primary inoculation, and again 7 days after a virus challenge. The virus-specific IgG1, IgG2a, and IgG2b levels were significantly different in the beta 2m-/- group (20-, 9-, and 30-fold lower, respectively, on day 7 after challenge) compared with the beta 2m+/- group. Virus-specific serum IgM levels for both groups remained similar throughout the experiment. In a separate experiment, beta 2m-/- mice were immunized with a nonviral antigen, 2,4,6-trinitrophenyl-conjugated keyhole limpet hemocyanin, and both total and antigen-specific isotype-specific immunoglobulin titers were determined. Total IgG1, IgG2a, IgG2b, and IgG3 tended to be lower overall in the beta 2m-/- mice compared with beta 2m+/- littermates. In contrast, total and antigen-specific IgE titers were similar in the two groups. These data indicate that CD8+ lymphocytes are not required to clear high doses of vaccinia virus, and they suggest that beta 2m-/- mice are less efficient at antigen-specific IgG production than their beta 2m+/- littermates.

The Steel factor.

Steel factor (SLF) is a recently identified growth factor which is the gene product of the murine Steel locus and a ligand for the c-kit tyrosine kinase receptor, the product of the dominant white spotting locus (W). Defects at these genetic loci result in aberrant melanocyte, germ cell, and hematopoietic development. Both the receptor (c-kit) and the ligand (SLF) have been shown to undergo tissue-specific mRNA splicing to produce distinct isoforms which have unique biological functions. As predicted by the phenotype of these mutations, SLF influences the growth and differentiation of melanocytes, primordial germ cells, and a broad spectrum of cell types in the hematopoietic progenitor and stem cell hierarchy. SLF has also been shown to have effects on hematopoietic lineages not predicted by defects seen in the Steel mouse.