Emergence of intrinsic superconductivity below 1.178 K in the topologically non-trivial semimetal state of CaSn3

Topological materials which are also superconducting are of great current interest, since they may exhibit a non-trivial topologically-mediated superconducting phase. Although there have been many reports of pressure-tuned or chemical-doping-induced superconductivity in a variety of topological materials, there have been few examples of intrinsic, ambient pressure superconductivity in a topological system having a stoichiometric composition. Here, we report that the pure intermetallic CaSn₃ not only exhibits topological fermion properties, but also has a superconducting phase at ~1.178 K under ambient pressure. The topological fermion properties, including the nearly zero quasi-particle mass and the non-trivial Berry phase accumulated in cyclotron motions, were revealed from the de Haas-van Alphen (dHvA) quantum oscillation studies of this material. Although CaSn₃ was previously reported to be superconducting with T _c  =  4.2 K, our studies show that the T _c  =  4.2 K superconductivity is extrinsic and caused by Sn on the degraded surface, whereas its intrinsic bulk superconducting transition occurs at 1.178 K. These findings make CaSn₃ a promising candidate for exploring new exotic states arising from the interplay between non-trivial band topology and superconductivity, e.g. topological superconductivity (TSC).

Electrostatic tuning of the proximity-induced exchange field in EuS/Al bilayers

We demonstrate that the proximity-induced exchange field H(ex) in ferromagnetic-paramagnetic bilayers can be modulated with an electric field. An electrostatic gate arrangement is used to tune the magnitude of H(ex) in the Al component of EuS/Al bilayers. In samples with H(ex)~2 T, we were able to produce modulations of ±10 mT with the application of perpendicular electric fields of the order of ±10(6) V/cm. We discuss several possible mechanisms accounting for the electric field's influence on the interfacial coupling between the Al layer and the ferromagnetic insulator EuS, along with the prospects of producing a superconducting field-effect transistor.

Exchange field-mediated magnetoresistance in the correlated insulator phase of Be films

We present a study of the proximity effect between a ferromagnet and a paramagnetic metal of varying disorder. Thin beryllium films are deposited onto a 5 nm thick layer of the ferromagnetic insulator EuS. This bilayer arrangement induces an exchange field, H(ex), of a few tesla in low-resistance Be films with sheet resistance R≪R(Q), where R(Q)=h/e2 is the quantum resistance. We show that H(ex) survives in very high-resistance films and, in fact, appears to be relatively insensitive to the Be disorder. We exploit this fact to produce a giant low-field magnetoresistance in the correlated-insulator phase of Be films with R≫R(Q).

Origin of excess low-energy states in a disordered superconductor in a Zeeman field

Tunneling density of states measurements of disordered superconducting Al films in high Zeeman fields reveal a significant population of subgap states which cannot be explained by standard BCS theory. We provide a natural explanation of these excess states in terms of a novel disordered Larkin-Ovchinnikov phase that occurs near the spin-paramagnetic transition at the Chandrasekhar-Clogston critical field. The disordered Larkin-Ovchinnikov superconductor is characterized by a pairing amplitude that changes sign at domain walls. These domain walls carry magnetization and support Andreev bound states that lead to distinct spectral signatures at low energy.

Spin-resolved tunneling studies of the exchange field in EuS/Al bilayers

We use spin-resolved electron tunneling to study the exchange field in the Al component of EuS/Al bilayers, in both the superconducting and normal-state phases of the Al. Contrary to expectation, we show that the exchange field H(ex) is a nonlinear function of applied field, even in applied fields that are well beyond the EuS coercive field. Furthermore, the magnitude H(ex) is unaffected by the superconducting phase. In addition, H(ex) decreases significantly with increasing temperature in the temperature range of 0.1-1 K. We discuss these results in the context of recent theories of generalized spin-dependent boundary conditions at a superconductor-ferromagnet interface.

Crystal growth, structure, and physical properties of Ln(Ag, Al, Si)₂ (Ln = Ce and Gd)

Single crystals of CeM₂ and GdM₂ (M = Ag, Al, and Si) were grown by the flux growth technique and characterized by means of single crystal x-ray diffraction, magnetic susceptibility, resistivity, and heat capacity measurements. CeM₂ and GdM₂ crystallize in the tetragonal I4(1)/amd space group with the α-ThSi₂ structure type with lattice parameters a ~4.2 Å and c ~14.4 Å. Curie-Weiss behavior is observed for both analogues with CeM₂ ordering first ferromagnetically at 11 K with a second antiferromagnetic transition at 8.8 K while GdM₂ orders antiferromagnetically at 24 K. Heat capacity measurements on CeM₂ show two magnetic transitions at 10.8 and 8.8 K with an electronic specific heat coefficient, γ(0), of ~53 mJ K(-2) mol(-1). The entropy at the magnetic transition is less than the expected Rln2 for CeM₂, reinforcing the assertions of an enhanced mass state and Kondo behavior being observed in the resistivity.

Saturation of the anomalous Hall effect in critically disordered ultrathin CNi3 films

We demonstrate that a distinct high-disorder anomalous Hall effect phase emerges at the correlated insulator threshold of ultrathin, amorphous, ferromagnetic CNi3 films. In the weak-localization regime, where the sheet conductance G>>e{2}/h, the anomalous Hall resistance of the films increases with increasing disorder and the Hall conductance scales as G{xy} proportional to G{phi} with phi=1.6. However, at sufficiently high disorder the system begins to enter the 2D correlated insulator regime, at which point the Hall resistance R{xy} abruptly saturates and the scaling exponent becomes phi=2. Tunneling measurements show that the saturation behavior is commensurate with the emergence of the 2D Coulomb gap, suggesting that e-e interactions mediate the high-disorder phase.

Crystal growth, structure, and physical properties of Ln(Cu,Al)12 (Ln = Y, Ce, Pr, Sm, and Yb) and Ln(Cu, Ga)12 (Ln = Y, Gd-Er, and Yb)

Single crystals of Ln(Cu,Al)12 and Ln(Cu,Ga)12 compounds (Ln = Y, Ce-Nd, Sm, Gd-Ho, and Yb for Al and Ln = Y, Gd-Er, Yb for Ga) have been grown by flux-growth methods and characterized by means of single-crystal x-ray diffraction, complemented with microprobe analysis, magnetic susceptibility, resistivity and heat capacity measurements. Ln(Cu,Ga)12 and Ln(Cu,Al)12 of the ThMn12 structure type crystallize in the tetragonal I4/mmm space group with lattice parameters a approximately 8.59 Å and c approximately 5.15 Å and a approximately 8.75 Å and c approximately 5.13 Å for Ga and Al containing compounds, respectively. For aluminium containing compounds, magnetic susceptibility data show Curie-Weiss paramagnetism in the Ce and Pr analogues down to 50 K with no magnetic ordering down to 3 K, whereas the Yb analogue shows a temperature-independent Pauli paramagnetism. Sm(Cu,Al)12 orders antiferromagnetically at T(N)approximately 5 K and interestingly exhibits Curie-Weiss behaviour down to 10 K with no Van Vleck contribution to the susceptibility. Specific heat data show that Ce(Cu,Al)12 is a heavy fermion antiferromagnet with T(N) approximately 2 K and with an electronic specific heat coefficient γ0 as large as 390 mJ K2 mol(-1). In addition, this is the first report of Pr(Cu,Al)12 and Sm(Cu,Al)12 showing an enhanced mass (approximately 80 and 120 mJ K(2) mol(-1)). For Ga containing analogues, magnetic susceptibility data also show the expected Curie-Weiss behaviour from Gd to Er, with the Yb analogue being once again a Pauli paramagnet. The antiferromagnetic transition temperatures range over 12.5, 13.5, 6.7, and 3.4 K for Gd, Tb, Dy, and Er. Metallic behaviour is observed down to 3 K for all Ga and Al analogues. A large positive magnetoresistance up to 150% at 9 T is also observed for Dy(Cu,Ga)12. The structure, magnetic, and transport properties of these compounds will be discussed.

Measurement of conduction-electron polarization via the pairing resonance

We show that the pairing resonance in the Pauli-limited normal state of ultrathin superconducting Al films provides a spin-resolved probe of conduction-electron polarization in thin magnetic films. A superconductor-insulator-ferromagnet tunneling junction is used to measure the density of states in supercritical parallel magnetic fields that are well beyond the Clogston-Chandresekhar limit, thus greatly extending the field range of the tunneling density of states technique. The applicability and limitations of using the pairing resonance as a spin probe are discussed.

Comment on "Post-Wildfire Logging Hinders Regeneration and Increases Fire Risk"

Donato et al. (Brevia, 20 January 2006, p. 352) concluded that logging after wildfire kills natural regeneration and increases fire risk. We argue that their paper lacks adequate context and supporting information to be clearly interpreted by scientists, resource managers, policy-makers, and the public.

Spin proximity effect in ultrathin superconducting Be-Au bilayers

We present a detailed study of the effects of interface spin-orbit coupling on the critical field behavior of ultrathin superconducting Be/Au bilayers. Parallel field measurements were made in bilayers with Be thicknesses in the range of d=2-30 nm and Au coverages of 0.5 nm. Though the Au had little effect on the superconducting gap, it produced profound changes in the spin states of the system. In particular, the parallel critical field exceeded the Clogston limit by an order of magnitude in the thinnest films studied. In addition, the parallel critical field unexpectedly scaled as [FORMULA: SEE TEXT], suggesting that the spin-orbit coupling energy was proportional to Delta0/d2. Tilted field measurements showed that, contrary to recent theory, the interface spin-orbit coupling induces a large in-plane superconducting susceptibility but only a very small transverse susceptibility.

Orbital response of evanescent cooper pairs in paramagnetically limited Al films

We report a detailed study of the pairing resonance via tunneling density of states in ultrathin superconducting Al films in supercritical magnetic fields. Particular emphasis is placed on the effects of the perpendicular component of the magnetic field on the resonance energy and magnitude. Though the resonance is broadened and attenuated by H(perpendicular) as expected, its energy is shifted upward linearly with H(perpendicular). Extension of the original theory of the resonance to include strong perpendicular fields shows that at sufficiently large H(perpendicular) the overlap of the broadened resonance tail with the underlying degenerate Fermi sea alters the spectral distribution of the resonance via the exclusion principle. This leads to the shift of the resonance feature to higher energy.

Low-temperature susceptibility of the noncentrosymmetric superconductor CePt3Si

We report ac susceptibility measurements of polycrystalline CePt(3)Si down to 60 mK and in applied fields up to 9 T. In a zero applied field, a full Meissner state emerges at temperatures T/T(c) < 0.3, where T(c) = 0.65 K is the onset transition temperature. Though transport measurements show a relatively high upper critical field B(c2) approximately 4-5 T, the low-temperature susceptibility chi(') is quite fragile to the applied field, with chi(') diminishing rapidly in fields of a few kG. Interestingly, the field dependence of chi(') is well described by the power law 4pichi(') + 1 = (B/B(c))(1/2), where B(c) is the field at which the onset of resistance is observed in transport measurements.

Field-induced spin mixing in ultrathin superconducting Al and be films in high parallel magnetic fields

We report spin-dependent electron density of states (DOS) studies of ultrathin superconducting Al and Be films in high parallel magnetic fields. Superconductor-insulator-superconductor (SIS) tunneling spectra are presented in which both the film and the counterelectrode are in the paramagnetic limit. This SIS configuration is exquisitely sensitive to spin mixing and/or spin flip processes which are manifest as DOS singularities at eV=2 Delta(0)+/-eV(z). Both our Al and Be data show a well defined subgap peak whose magnitude grows dramatically as the parallel critical field is approached. Though this feature has previously been attributed to spin-orbit scattering, it is more consistent with fluctuations into a field induced mixed-spin state.

Quantum metallicity in a two-dimensional insulator

One of the most far-reaching problems in condensed-matter physics is to understand how interactions between electrons, and the resulting correlations, affect the electronic properties of disordered two-dimensional systems. Extensive experimental and theoretical studies have shown that interaction effects are enhanced by disorder, and that this generally results in a depletion of the density of electronic states. In the limit of strong disorder, this depletion takes the form of a complete gap in the density of states. It is known that this 'Coulomb gap' can turn a pure metal film that is highly disordered into a poorly conducting insulator, but the properties of these insulators are not well understood. Here we investigate the electronic properties of disordered beryllium films, with the aim of disentangling the effects of the Coulomb gap and the underlying disorder. We show that the gap is suppressed by a magnetic field and that this drives the strongly insulating beryllium films into a low-temperature 'quantum metal' phase with resistance near the quantum resistance RQ = h/e2, where h is Planck's constant and e is the electron charge.

Tenfold magnetoconductance in a nonmagnetic metal film

We present magnetoconductance (MC) measurements of homogeneously disordered Be films whose zero field sheet conductance ( G) is described by the Efros-Shklovskii hopping law G(T) = (2e(2)/h)exp-(T0/T)(1/2). The low field MC of the films is negative with G decreasing a factor of 2 below 1 T. In contrast the MC above 1 T is strongly positive. At 8 T, G increases tenfold in perpendicular field and fivefold in parallel field. In the simpler parallel case, we observe field enhanced variable range hopping characterized by an attenuation of T0 via the Zeeman interaction.

Coulomb gap: how a metal film becomes an insulator

Electron tunneling measurements of the density of states (DOS) in ultrathin Be films reveal that a correlation gap mediates their insulating behavior. In films with sheet resistance R<5000 Omega the correlation singularity appears as the usual perturbative ln(V) zero bias anomaly (ZBA) in the DOS. As R is increased further, however, the ZBA grows and begins to dominate the DOS spectrum. This evolution continues until a nonperturbative |V| Efros-Shklovskii Coulomb gap spectrum finally emerges in the highest R films. Transport measurements of films which display this gap are well described by a universal variable range hopping law R(T) = (h/2e(2))exp(T0/T)(1/2).

Comparison of crossmatch results obtained by ELISA, flow cytometry, and conventional methodologies

The complement-dependent lymphocytotoxicity crossmatch (CXM) is the presently accepted standard for detection of donor-reactive alloantibodies in transplant patients. However, the newer flow cytometric (FXM) and ELISA (EXM) crossmatch technologies are increasingly used as substitutes for the CXM. We have compared the sensitivity and reproducibility of FXM vs. EXM and, in general, find them to be quite similar. However, when we compared the agreement of FXM vs. EXM in 112 donor/recipient combinations, we found that they identified different subsets of donor-specific alloantibodies in about 35% of the tests. When compared to the standard CXM method, the EXM correlated much better than did the FXM, yielding a much lower rate of false positive (2.5% vs. 8%) and false negative (7% vs. 18.5%) results. The reduction in time required to obtain a result (3 h) and the cost of materials ($25/test) was identical for the EXM and FXM. We conclude that the ELISA method for crossmatching has advantages over the flow cytometric method as a substitute for the present standard complement-dependent lymphocytotoxicity method.

Trans vivo analysis of human delayed-type hypersensitivity reactivity

There are clinical situations in which it may be advantageous to monitor delayed type hypersensitivity (DTH) responses, an index of cell-mediated immunity, without exposing patients directly to the challenge antigens. For example, transplant patients may be at risk for becoming sensitized to donor antigens if injected with donor antigen during traditional skin tests. We describe an alternative method for human DTH testing, which involves the transfer of human peripheral blood mononuclear cells plus antigen into the pinnae or footpads of naive mice. This induces a measurable DTH-like swelling response, which we refer to as the "trans vivo DTH response." As proof of principle, we provide data obtained during trans vivo DTH studies with tetanus toxoid, cytomegalovirus (CMV) and alloantigens. In general, human T cells must be co-localized with antigen and human macrophages to produce swelling responses, and such responses are antigen-specific and require prior antigen sensitization. Not only does this assay offer a simple, reliable clinical monitoring device, but it also provides a model with which to study the in vivo mechanisms of human DTH responses.

Cytolytic activity against allogeneic human endothelia: resistance of cytomegalovirus-infected cells and virally activated lysis of uninfected cells

BACKGROUND

Cytomegalovirus (CMV) has been implicated as an exacerbating agent in the development of transplant vascular sclerosis; however, specific etiologic mechanisms remain unresolved. Based upon our previous observations that CMV-infected endothelial cells (ECs) stimulate proliferation and cytokine production by allogeneic T cells, we now test the hypothesis that CMV-driven cytolytic activity may contribute to graft endothelial injury.

METHODS

Limiting dilutions of CMV-seropositive or -seronegative donor-derived T cells were stimulated with CMV-infected or uninfected allogeneic ECs in the presence of interleukin-2. T-cell proliferation was monitored by assay of [3H]thymidine incorporation and stimulated T cells were tested for lytic activity against CMV-infected or uninfected radiolabeled EC targets by 51Cr release assay. Natural killer (NK) cell activity was examined by incubating freshly isolated peripheral blood mononuclear cells with 51Cr-labeled targets, followed by assay of radiolabel release.

RESULTS

CMV-infected ECs were resistant to T cell- and NK-mediated cytolysis regardless of donor serostatus, nature of stimulation, or level of T-cell proliferation. In contrast, although uninfected ECs were unharmed by NK cells, these targets experienced significant lysis by T cells stimulated with either uninfected or CMV-infected ECs.

CONCLUSIONS

These results implicate CMV-infected graft endothelium as a persistent source of infectious virus, a chronic stimulus for potentially destructive host inflammatory activity, and a potential trigger for the generation of lytic injury to uninfected bystander endothelia, suggesting multiple mechanisms by which this virus might perturb equilibrium at the graft/host interface.

Transplantation immunology

The practice of clinical and experimental transplantation continues to evolve at a rapid pace. To appreciate the current transplant practices, it is first necessary to review transplant immunology in its proper context, ie, as a component of the complex series of events that promote the repair of damaged tissues. These processes are generally categorized as inflammation, immunity, and tissue repair/reinforcement. In general, there are 3 forms of graft rejection: hyperacute, acute, and chronic rejection. All 3 forms of graft rejection represent pathologic consequences of one or more of these repair-related processes. The various graft rejection responses also illustrate several complex immunologic principles that need to be considered. These include the definition of an alloantigen, the structure and function of major histocompatibility complex molecules, and the behavior of antigen-presenting cells and alloreactive T cells. This review combines these concepts and principles into a discussion of the 3 forms of graft rejection, each of which is addressed at the level of histopathology, pathobiology, incidence, and clinical strategies.

Clinical and economic impact of flow cytometry crossmatching in primary cadaveric kidney and simultaneous pancreas-kidney transplant recipients

We retrospectively compared the clinical and financial impact of a final cross-match by T cell flow cytometry (FXM) versus conventional complement-dependent cytotoxicity (CXM) in consecutive primary cadaveric kidney (K) and primary simultaneous cadaveric pancreas-kidney (SPK) transplant recipients. Mean follow-up was 14 months for both the K (range, 5-22 months) and SPK (range, 5-22 months) recipients. There were no instances of a positive CXM result if the FXM result was negative. However, 18 of the 102 (18%) K recipients and 11 of the 66 (17%) SPK recipients were FXM positive, CXM negative, but no grafts lost to hyperacute rejection in this group. In addition, patient survival, graft survival, incidence of acute rejection, and kidney and pancreas function (immediate and late) were not different in the FXM-positive versus the FXM-negative groups. Charges for the CXM and FXM methods were compared over a 6-month period. During that period, the FXM charges averaged $583 less per recipient than the CXM charges (58% reduction in charges), and the time required to perform the FXM method was 50% of that required for the CXM method. These results demonstrate that a clinical pathway for primary transplantation that utilizes the FXM rather than the CXM final cross-match is clinically safe, with no adverse effect on posttransplant outcome, reduces organ preservation time by shortening the waiting period for the final cross-match results, and significantly reduces the tissue typing charges. However, about 9% of all primary K and SPK recipients will be FXM positive, CXM negative on final cross-match and will be unnecessarily denied a transplant. In this study, we describe a method to identify these patients so that they can be tested by traditional CXM to avoid being denied access to donor organs.

Transperineal ultrasound-guided radioactive seed implantation for organ-confined carcinoma of the prostate

PURPOSE

This retrospective study was undertaken to: (a) determine the prognostic significance of pretreatment and 1-year nadir serum prostate specific antigen (PSA) levels in organ-confined carcinoma of the prostate treated with ultrasound-guided radioactive 125I seed implantation; (b) determine the factors associated with postimplant morbidity and whether modification of the technique would reduce morbidity; (c) evaluate the local control rate and disease-free survival of patients undergoing seed implantation.

METHODS AND MATERIALS

From October 1988 through December 1992, 142 patients with organ-confined adenocarcinoma of the prostate and a Gleason score < or = 7 underwent ultrasound-guided radioactive 125I seed implantation as an alternative to radical prostatectomy. Patients were considered to have persistent or progressive disease if there was evidence of local progression on digital exam, or if there were two consecutive increases in the PSA level. Patients suspected of persistent or progressive disease underwent restaging to include CT scan of the pelvis, bone scan, and ultrasound-guided prostate biopsy. Patients with increasing PSA levels in which active disease could not be confirmed were considered biochemical failures with occult systemic disease and were offered hormone ablation.

RESULTS

With 1-6-year follow-up, median 30 months, the relapse patterns were prostate 4 (2.8%), bone 4 (2.8%), rising PSA 16 (11%). Pretreatment PSA level correlated with subsequent recurrence; pretreatment PSA < or = 4 (0), 4.1 to 10 (14%), 10.1 to 20 (21%), 20.1 to 50 (58%). Disease free survival at 2 years was 90% and at 5 years 76%. Nadir PSA (nPSA) at 1 year also correlated with recurrence: nPSA < or = 1 (3%), nPSA 1 < or = 4 (50%), and nPSA > or = 4 (100%). Seed implantation was well tolerated with 31% of patients experiencing RTOG morbidity > or = Grade 2, which typically consisted of transient radiation urethritis, which resolved with conservative measures. Eleven (8%) experienced RTOG morbidity > or = Grade 3. There was no correlation between number of seeds or total millicuries implanted and subsequent morbidity. However, reduction in the periurethral seed intensity reduced > or = Grade 3 morbidity from 11 to 4%.

CONCLUSION

Ultrasound-guided radioactive seed implantation provides excellent local control of 97%, with a median 30 month follow-up. Morbidity is comparable to other curative modalities and by modifying Blasko's technique to reduce radioactive seed strength in the periurethral area, significant morbidity is rare. Pretreatment PSA and the nadir PSA at 1 year are important predictors of subsequent disease outcome. With a liberal definition of systemic recurrence as two consecutive increases in PSA levels, the 5-year disease-free survival is 76%.

Nonradioactive alternative to clinical mixed lymphocyte reaction

The MLR is used clinically as a functional assay for genetic HLA disparity. Traditionally this test relies on [3H]thymidine incorporation to detect T-cell proliferation as an indicator of alloantigenicity. Environmental and administrative concerns regarding radioisotope use and disposal have encouraged the development of sensitive, nonradioactive assay for T-cell alloactivation. We describe a nonradioactive alternative to the clinical MLR which uses an IL-2-dependent cell line, CTLL20.3, and MTT reduction to detect IL-2 accumulation in MLC SNs as an index of T-cell alloactivation. We first determined (a) the optimal number of CTLL20.3 cells for the assay, (b) the optimal time of SN analysis for IL-2, and (c) additional manipulations that significantly increase the assay sensitivity for IL-2. Using this assay system with patient lymphocyte combinations, we demonstrated that the CTLL20.3-MTT assays correlate well with the [3H]thymidine assays of T-cell proliferation for the detection of MHC incompatibility. Indeed, the CTLL20.3-MTT assay may be slightly more sensitive than the traditional clinical MLR.

Alloantigenicity of human endothelial cells. III. Quantitated indirect presentation of endothelial alloantigens to human helper T lymphocytes

We have investigated the contribution of monocytes (Mo) to the activation of purified human T cells by allogeneic human vascular endothelial cells (HUVEC). We have previously demonstrated that allogeneic HUVEC stimulate IL-2 production by CD8+ helper T lymphocytes (HTL), but not CD4+ HTL, in the absence of accessory Mo. We now show that addition of responder-autologous Mo to such cultures stimulates a high frequency of CD4+ HTL (1/6500), but no additional CD8+ HTL (1/35,000), as detected by limiting dilution analysis (LDA). The CD4+ HTL production of IL-2 increased with increasing numbers of Mo. Monoclonal antibodies to MHC class II interfered with HTL responses to allogeneic HUVEC in the presence, but not in the absence of autologous Mo. In contrast, IFN-gamma-treated HUVEC stimulated a high frequency of CD4+ HTL in the absence of autologous Mo. However, deletion experiments revealed that the population of HTL responsive to IFN-gamma-treated HUVEC is distinct from the population that responds to HUVEC in the presence of autologous Mo. These data suggest that Mo promote IL-2 production by presenting HUVEC-derived alloantigens via MHC class II molecules to CD4+ HTL, rather than by providing cytokines that promote more efficient IL-2 production by CD8+ HTL, or by inducing MHC class II expression on the HUVEC. In general, these data demonstrate that autologous Mo can play a significant role in the response of T cells to allogeneic HUVEC. Further, they demonstrate that the activation of human HTL by allogeneic HUVEC is complex, and can occur by at least three pathways: (1) direct stimulation of a small number of CD8+ HTL, but no CD4+ HTL, by quiescent HUVEC, (2) direct stimulation of a large number of CD4+ HTL by IFN-gamma-treated HUVEC, and (3) indirect stimulation of a different subset of CD4+ HTL by HUVEC in the presence of autologous monocytes. These three pathways of alloactivation are not unique to allogeneic HUVEC, but this experimental system provides a convenient and relevant model with which each pathway can be easily and independently investigated.

HLA-B44-directed cytotoxic T cells associated with acute graft-versus-host disease following unrelated bone marrow transplantation

We describe the recipient of a marrow graft from an HLA-serologically identical unrelated donor from whom highly potent host-reactive CTL of donor origin were isolated in association with acute GVHD. Extensive sequence and biochemical analysis of the HLA complex of this donor and recipient revealed several disparities in class I and class II HLA with the potential to be recognized by T cells from the donor or the host. The donor-derived CTL exclusively recognized a class I HLA difference associated with HLA-B44. Nucleotide sequencing of donor and recipient cells revealed that the patient possessed the HLA-B*4402 allele recognized by IEF as B44.2 while the donor possessed HLA-B*4403 (IEF variant B44.1). These alleles differ at one amino acid residue located at position 156 in the alpha 2 domain. The donor-derived CTL were shown to be specific for B44.2 by blocking studies and by the lysis of five different B44.2+ unrelated cell lines, two of which were confirmed by sequencing to be homozygous for B*4402. A host-specific difference involving a HLA-DRB1 allele was not recognized by the CTL, neither did HLA differences unique to the donor HLA-B*4403 and HLA-DQ8 elicit a host response. These data show that certain HLA disparities may be tolerated at the same time that other disparities elicit a potent immunologic response. The chemical nature of the difference, its structural impact, as well as the conditions of transplant appear to influence the type of response which occurs.

Alloantigenicity of human endothelial cells. II. Analysis of interleukin 2 production and proliferation by T cells after contact with allogeneic endothelia

In organ allograft recipients, the first point of contact between the host immune system and the graft alloantigens is the graft vascular endothelia. Our previous experiments have established that there are multiple pathways by which T cells can be activated in vitro by endothelial alloantigens. In this report, we focus on the first of these pathways: the direct activation of T cells by endothelial MHC class I molecules. Using conventional mixed cell cultures and limiting dilution analyses, we demonstrate that "resting" allogeneic human umbilical vein endothelial cells (HUVEC) stimulate IL-2 production, but not proliferation of purified CD3+ PBMC. Transwell experiments demonstrate that soluble suppressive factors are not responsible for the lack of proliferation. Instead, they suggest that the production of growth factors, such as IL-2, is suboptimal in this system. Indeed, submitogenic concentrations of IL-2 synergize with HUVEC to induce strong T cell proliferative responses. This proliferation is associated with a detectable increase in T cell IL-2R expression, which is not apparent after stimulation with HUVEC or IL-2 alone. In conjunction with previous data, these observations characterize the first direct pathway of endothelia-induced T cell activation. Via this pathway, a small number of CD8+ T cells can be activated by the allogeneic MHC class I molecules displayed by resting allogeneic endothelial cells. This activation results in the elaboration of IL-2, among other things, in concentrations that are too small to promote IL-2R up-regulation, and thus T cell proliferation. Proliferation readily occurs if sufficient IL-2 is available in the environment to overcome this cytokine deficit. These studies suggest that endothelial MHC class I alloantigens are mildly antigenic to a small subset of T cells. However, in an inflammatory environment that is rich in cytokines and growth factors, these endothelial alloantigens may become potent direct stimulators of T cell activation and clonal expansion.

In vitro induction of endothelial HLA class II antigen expression by cytomegalovirus-activated CD4+ T cells

CMV has been associated with allograft rejection and transplantation-associated arteriosclerosis. CMV infects endothelium, the interface between allograft tissue and the host immune system. Although endothelial HLA class II expression is a hallmark of vascular rejection, CMV does not directly induce these antigens on infected endothelial cells (EC) and, indeed, renders them refractory to HLA DR induction by IFN-gamma. Our earlier studies have demonstrated, however, that CMV-infected EC are capable of eliciting vigorous activation responses by allogeneic, CMV-seropositive donor-derived CD4+ T cells. We now show that T cells thus activated can induce HLA DR expression on noninfected EC. Human umbilical vein endothelial cells (HUVEC) were inoculated at low titer with CMV strain VHL/E, cocultured with allogeneic, CMV-seropositive or CMV-seronegative donor-derived CD4+ T cells, then dual immunohistochemically stained for CMV antigen and HLA DR, or assayed for HLA DR expression by fluorescence flow cytometry. Results demonstrated that HLA DR was induced in 20-70% of HUVEC in monolayers containing less than 0.5% CMV-infected EC after coculture with CMV-seropositive donor-derived T cells. No such induction was observed in experiments employing T cells isolated from CMV-seronegative individuals. Expression of this antigen was strictly limited to noninfected cells. Rare HLA DR induction was observed in virus-free cocultures. To determine whether endothelial HLA DR was induced by a soluble factor(s) elaborated by activated T cells, noninfected HUVEC monolayers were treated for 72 hr with cell-free supernatant from CMV-infected or noninfected HUVEC/T cell cocultures and assayed as above. Again, HLA DR expression was induced by supernatant from CMV-positive cocultures (only when cocultured T cells were isolated from CMV-seropositive donors), but not by supernatant from CMV-negative cocultures or from T cells cultured alone. The soluble factor was identified as IFN-gamma by the complete attenuation of HLA DR induction by anti-IFN-gamma mAb. These findings suggest that allograft endothelium harboring low level CMV may be capable of eliciting host CD4+ T cell activation, and that consequent release of IFN-gamma is capable of inducing endothelial HLA class II expression, as observed in vascular rejection and transplantation-associated arteriosclerosis. Results of these studies thus provide insight into mechanisms that may help elucidate the association between CMV and rejection-related immune events in the allograft.

T lymphocyte activation by cytomegalovirus-infected, allogeneic cultured human endothelial cells

Cytomegalovirus, a source of serious complications among immunosuppressed individuals, infects endothelial cells in vivo, and has been epidemiologically associated with allograft rejection and transplantation-associated accelerated arteriosclerosis (TxAA). As the interface between the host immune system and allograft tissues, the endothelium may be of primary importance in mediating pathogenic immune responses to CMV. We have therefore investigated T lymphocyte responses to CMV-infected allogeneic human umbilical vein endothelial cells (HUVE) in vitro. Proliferation assays demonstrated dramatically enhanced responses by CMV-seropositive donor-derived PBMC or purified T cells cocultured with CMV-infected HUVE, as compared with those elicited by noninfected stimulator cells. As determined by limiting dilution analysis of IL-2-producing cells, the frequency of T cells responding to infected HUVE was generally found to exceed by approximately one order of magnitude those responding to uninfected cells. Similar analyses of isolated T cell subsets revealed that these responses (proliferation and IL-2 production) were nearly entirely accountable to the CD4+ fraction. Responses of CD8+ populations, however, varied among donors. The marked activation of CD4+ cells is particularly intriguing since we have shown that CMV-infected HUVE do not express HLA class II antigens. Responses of lymphocytes derived from CMV-seronegative donors were minimal in all assays. These studies show that purified T cells can respond to CMV in the exclusive context of allogeneic endothelial cells. Furthermore, we demonstrate the heretofore undescribed phenomenon of CD4+ T cell activation in the absence of serologically detectable HLA class II antigen. As vascular rejection and TxAA are characterized by subendothelial T cell infiltration, these findings support a role for CMV/endothelial interaction in their pathogenesis.

Evaluation of an automated method of percent reactive antibody determination

A fluorescence-based automated method of percent reactive antibody (PRA) analysis is described. This method utilizes the conventional antibody-mediated, C'-dependent lymphocyte microcytotoxicity assay to detect alloantibodies, but replaces the eosin-based method for detection of cell death with a fluorescence-based method. To identify viable cells, lymphocytes were pretreated with carboxy fluorescein diacetate (CFDA), which fluoresces green, to identify viable cells. To identify dead cells after the reaction with antibody and C', they were treated with propidium iodide (PI), which fluoresces red. Pretreatment of lymphocytes with CFDA did not affect their ability or interact with alloantibodies in the microcytotoxicity assays. When visually analyzed, detection of cell death by fluorescence was as sensitive as detection by eosin exclusion. However electronic detection of fluorescence was slightly more sensitive than visual detection. Automation of the fluorescent method required a calculation that converts electronic data to an ASHI score for cell death. One such method is described and evaluated. Both the automated and the conventional methods of analysis were used to obtain PRA values for various sera. There was good correlation between the PRA values obtained with the automated method versus the conventional method. Further, there was good correlation for PRA-derived alloantibody specificities obtained with the automated method versus the conventional method. These data demonstrate that automated fluorescence-based PRA analysis is an effective and practical alternative to conventional PRA analysis.

Alloantigenicity of human endothelial cells. 1. Frequency and phenotype of human T helper lymphocytes that can react to allogeneic endothelial cells

To determine the relative ability of allogeneic endothelial cells to stimulate helper T lymphocytes (HTL), human PBMC or purified T cells were incubated in conventional lymphocyte microcultures or in limiting dilution microcultures with allogeneic human umbilical vein endothelia (HUVE), with cytokine-treated allogeneic HUVE, or with allogeneic peripheral blood monocytes. These cultures were tested for IL-2 production as an index of HTL stimulation. Dose-response studies in conventional lymphocyte cultures indicated that allogeneic monocytes were better than allogeneic HUVE at stimulating IL-2 production. Limiting dilution analyses revealed that untreated HUVE and TNF-treated HUVE stimulated small numbers of HTL (approximately 1 HTL/30,000 PBMC), whereas 5 to 10 times more HTL were stimulated by IFN-gamma-treated HUVE and 10 to 20 times more HTL were stimulated by allogeneic monocytes. Serologic deletion studies revealed that most of the high frequency HTL responding to IFN-gamma-treated HUVE were CD4+, whereas most of the low frequency HTL responding to nontreated HUVE or to TNF-treated HUVE were CD8+. Interestingly, mAb to MHC class I and class II molecules, which significantly impaired HUVE-induced proliferation, caused little interference with HUVE-induced IL-2 production. Finally, polymerase chain reaction analysis demonstrated that untreated allogeneic HUVE cells could stimulate PBMC to produce mRNA for IFN-gamma, as well as for IL-2. These data demonstrate the following hierarchy of allogeneic stimulatory capacity for human HTL: monocytes greater than IFN-gamma-treated HUVE much greater than TNF-treated HUVE = nontreated HUVE. Further, these data suggest that non-activated allogeneic endothelial cells can initiate immune responses by inducing IL-2 and IFN-gamma. Because IFN-gamma can induce MHC class II expression by the endothelial cells, this could recruit large numbers of CD4+ T cells for IL-2 production.

Alloantigenicity of human endothelial cells. 1. Frequency and phenotype of human T helper lymphocytes that can react to allogeneic endothelial cells

To determine the relative ability of allogeneic endothelial cells to stimulate helper T lymphocytes (HTL), human PBMC or purified T cells were incubated in conventional lymphocyte microcultures or in limiting dilution microcultures with allogeneic human umbilical vein endothelia (HUVE), with cytokine-treated allogeneic HUVE, or with allogeneic peripheral blood monocytes. These cultures were tested for IL-2 production as an index of HTL stimulation. Dose-response studies in conventional lymphocyte cultures indicated that allogeneic monocytes were better than allogeneic HUVE at stimulating IL-2 production. Limiting dilution analyses revealed that untreated HUVE and TNF-treated HUVE stimulated small numbers of HTL (approximately 1 HTL/30,000 PBMC), whereas 5 to 10 times more HTL were stimulated by IFN-gamma-treated HUVE and 10 to 20 times more HTL were stimulated by allogeneic monocytes. Serologic deletion studies revealed that most of the high frequency HTL responding to IFN-gamma-treated HUVE were CD4+, whereas most of the low frequency HTL responding to nontreated HUVE or to TNF-treated HUVE were CD8+. Interestingly, mAb to MHC class I and class II molecules, which significantly impaired HUVE-induced proliferation, caused little interference with HUVE-induced IL-2 production. Finally, polymerase chain reaction analysis demonstrated that untreated allogeneic HUVE cells could stimulate PBMC to produce mRNA for IFN-gamma, as well as for IL-2. These data demonstrate the following hierarchy of allogeneic stimulatory capacity for human HTL: monocytes greater than IFN-gamma-treated HUVE much greater than TNF-treated HUVE = nontreated HUVE. Further, these data suggest that non-activated allogeneic endothelial cells can initiate immune responses by inducing IL-2 and IFN-gamma. Because IFN-gamma can induce MHC class II expression by the endothelial cells, this could recruit large numbers of CD4+ T cells for IL-2 production.

Detection of donor-reactive alloantibody in the early posttransplant period. Elimination of therapeutic ALG as a complicating factor

We have developed an immunoabsorbent reagent that can differentially remove ALG from human serum samples in vitro--i.e., goat antihorse IgG covalently linked to Sepharose beads. When used according to protocol, this immunoabsorbent can effectively remove up to 0.78 mg/ml of ALG from human serum mixtures. To demonstrate that immunoabsorption is selective, an HLA-B7-specific alloserum was mixed with a known amount of ALG and absorbed with antibody-conjugated Sepharose beads. The addition of ALG to the serum sample caused high degrees of nonspecific lympholysis in standard microcytotoxicity assays. When this serum/ALG mixture was immunoabsorbed detectable ALG activity was lost but the original HLA alloantibody titer (1:2) against specific target lymphocytes was retained.

Quantitation of antigen-specific immune responses in human immunodeficiency virus (HIV)-infected individuals by limiting dilution analysis

The lymphocyte proliferative response to recall antigens is lost following HIV infection. We sought to devise a means by which the functional immune status of persons in the early stages of HIV infection could be monitored quantitatively. The response to tetanus toxoid was examined in 45 HIV-infected individuals and 11 controls using conventional lymphocyte proliferative assays concurrently with limiting dilution analysis utilizing the secretion of interleukin-2 as the measure of a response. Our data show that the limiting dilution analysis detects tetanus toxoid-reactive T cells in 80% of those tested, as compared to only 44% by proliferation. However, the frequency of tetanus-reactive T cells in HIV-infected individuals (median frequency = 1/59,156) is decreased five-fold as compared to seronegative controls (median frequency = 1/11,599). Longitudinal studies demonstrated a time-dependent decrease in the frequency of tetanus-specific T cell responses in the HIV-infected individuals. Thus, the limiting dilution analysis is a quantitative approach for detecting antigen-specific T cells in HIV-infected individuals, and may be used to monitor changes in T cell function in HIV infection.

Limiting dilution analysis of human, tetanus-reactive helper T lymphocytes. A rapid method for the enumeration of helper T lymphocytes with specificity for soluble antigens

The number of helper T lymphocytes (HTL) in human peripheral blood with specificity for the soluble protein, tetanus toxoid, was estimated by limiting dilution analysis (LDA). HTL were detected via antigen-induced interleukin-2 (IL-2) production, as measured by incorporation of tritiated thymidine by an IL-2-dependent indicator cell line, CTLL-20. Culture conditions optimizing assay sensitivity are described, and the ability to detect antigen-specific HTL using this LDA technique are demonstrated. Observed HTL frequencies in healthy human donors tested for tetanus-reactive helper T cells ranged from less than 1 HTL/268,749 peripheral blood mononuclear cells (PBMC) (undetectable) to 1 HTL/1486 PBMC. The LDA technique was also used to detect frequency shifts in human peripheral blood HTL following challenge with antigen. This assay offers distinct advantages over proliferative LDA techniques in that it is rapid (requiring only 2 days), and defines an antigen-reactive T cell subset with defined function (IL-2 secretion). Furthermore, the LDA technique can be adapted for the detection of other soluble protein antigens, such as PPD and Candida albicans. In general, this LDA technique provides a reliable, quantitative index of human HTL reactivity to any of a variety of soluble protein antigens, and has clinical as well as experimental applicability.

Enumeration of autoreactive helper T lymphocytes in uveitis

In a one-stage, interleukin-2 (IL-2), limiting-dilution analysis, peripheral blood mononuclear cells from patients with uveitis and normal control subjects were assayed for S-antigen specific, tetanus-specific, and in vivo activated helper T cells. Controls subjects consistently demonstrated tetanus-specific responses, but neither in vivo activation nor S-antigen specific helper T cell responses were seen. Patients with active forms of diffuse, posterior, and anterior uveitis were found to have significant frequencies of both in vivo activated and S-antigen specific helper T cells in their peripheral blood. These data show that patients with certain forms of uveitis have a measurable frequency of lymphocytes in the peripheral immunologic compartment capable of secreting IL-2 in response to autologous presentation of ocular autoantigen (S-antigen). Limiting-dilution analysis techniques, generating minimal responder cell frequency estimates and distinct IL secretion patterns, may provide an index of disease activity and critical information about the mechanism(s) of ocular inflammation.

Enumeration of viral antigen-reactive helper T lymphocytes in human peripheral blood by limiting dilution for analysis of viral antigen-reactive T-cell pools in virus-seropositive and virus-seronegative individuals

A limiting-dilution analysis technique was developed which enumerates human T cells with the capacity to secrete T-cell growth factors such as interleukin 2 after contact with herpes simplex virus type 1 (HSV-1) or cytomegalovirus (CMV) antigens (operationally defined as virus-reactive helper T cells [HTL]). By using this limiting-dilution analysis technique, the peripheral blood of HSV-seropositive individuals was analyzed for the frequency of HSV antigen-reactive HTL and for the ability either to proliferate or to secrete detectable T-cell growth factors in conventional HSV antigen-stimulated lymphocyte cultures. We found that the magnitudes of the latter two responses did not correlate directly with the frequency estimates of HSV antigen-reactive HTL. The study was expanded to analyze both HSV and CMV reactivities within individuals. Those who were seropositive for HSV or CMV were found to have relatively high HTL frequencies for the viral antigens to which they were sensitized. However, those who were seronegative for one of the viruses often had HTL reactive with that virus in their peripheral blood. These latter HTL frequencies were highly variable and ranged from undetectable to quite prominent, even within the same individual at different times. In general, it was found that viral antigen-reactive serologic activity does not necessarily reflect the status of viral antigen-reactive cell-mediated immunity in humans and that viral antigen-induced T-cell responses may be unexpectedly complex, rather than absent, in individuals who are seronegative for a particular virus.