Understanding HIV infection for the design of a therapeutic vaccine. Part II: Vaccination strategies for HIV.

HIV infection leads to a gradual loss CD4(+) T lymphocytes comprising immune competence and progression to AIDS. Effective treatment with combined antiretroviral drugs (cART) decreases viral load below detectable levels but is not able to eliminate the virus from the body. The success of cART is frustrated by the requirement of expensive lifelong adherence, accumulating drug toxicities and chronic immune activation resulting in increased risk of several non-AIDS disorders, even when viral replication is suppressed. Therefore, there is a strong need for therapeutic strategies as an alternative to cART. Immunotherapy, or therapeutic vaccination, aims to increase existing immune responses against HIV or induce de novo immune responses. These immune responses should provide a functional cure by controlling viral replication and preventing disease progression in the absence of cART. The key difficulty in the development of an HIV vaccine is our ignorance of the immune responses that control of viral replication, and thus how these responses can be elicited and how they can be monitored. Part one of this review provides an extensive overview of the (patho-) physiology of HIV infection. It describes the structure and replication cycle of HIV, the epidemiology and pathogenesis of HIV infection and the innate and adaptive immune responses against HIV. Part two of this review discusses therapeutic options for HIV. Prevention modalities and antiretroviral therapy are briefly touched upon, after which an extensive overview on vaccination strategies for HIV is provided, including the choice of immunogens and delivery strategies.

Understanding HIV infection for the design of a therapeutic vaccine. Part I: Epidemiology and pathogenesis of HIV infection.

HIV infection leads to a gradual loss CD4+ T lymphocytes comprising immune competence and progression to AIDS. Effective treatment with combined antiretroviral drugs (cART) decreases viral load below detectable levels but is not able to eliminate the virus from the body. The success of cART is frustrated by the requirement of expensive life-long adherence, accumulating drug toxicities and chronic immune activation resulting in increased risk of several non-AIDS disorders, even when viral replication is suppressed. Therefore there is a strong need for therapeutic strategies as an alternative to cART. Immunotherapy, or therapeutic vaccination, aims to increase existing immune responses against HIV or induce de novo immune responses. These immune responses should provide a functional cure by controlling viral replication and preventing disease progression in the absence of cART. The key difficulty in the development of an HIV vaccine is our ignorance of the immune responses that control of viral replication, and thus how these responses can be elicited and how they can be monitored. Part one of this review provides an extensive overview of the (patho-) physiology of HIV infection. It describes the structure and replication cycle of HIV, the epidemiology and pathogenesis of HIV infection and the innate and adaptive immune responses against HIV. Part two of this review discusses therapeutic options for HIV. Prevention modalities and antiretroviral therapy are briefly touched upon, after which an extensive overview on vaccination strategies for HIV is provided, including the choice of immunogens and delivery strategies.

Sequence evolution and escape from specific immune pressure of an HIV-1 Rev epitope with extensive sequence similarity to human nucleolar protein 6.

Antigen-specific immunity is crucially important for containing viral replication in human immunodeficiency virus (HIV)-1-infected hosts. Several epitopes have been predicted for the early expressed HIV-1 proteins Tat and Rev, but few have been studied in detail. We characterized the human leukocyte antigen (HLA)-B44-restricted Rev epitope EELLKTVRL (EL9) in an HIV-1-infected subject treated with antiretroviral therapy. Interestingly, a high sequence similarity was found between the EL9 epitope and the human nucleolar protein 6 (NOL6). However, this similarity does not seem to impede immunogenicity as CD8(+) T-cells, previously stimulated with EL9-pulsed dendritic cells, were able to specifically recognize the HIV-1 Rev epitope without cross-recognizing the human self-antigen NOL6. After the subject interrupted antiretroviral therapy and virus rebounded, mutations within the EL9 epitope were identified. Although the emerging mutations resulted in decreased or abolished T-cell recognition, they did not impair Rev protein function. Mutations leading to escape from T-cell recognition persisted for up to 124 weeks following treatment interruption. This study shows that the HLA-B44-restricted Rev CD8(+) T-cell epitope EL9 is immunogenic notwithstanding its close resemblance to a human peptide. The epitope mutates as a consequence of dynamic interaction between T-cells and HIV-1. Clinical status, CD4(+) T-cell count and viral load remained stable despite escape from T-cell recognition.

Functional and phenotypic evidence for a selective loss of memory T cells in asymptomatic human immunodeficiency virus-infected men.

In addition to a well-documented depletion of CD4+ T helper cells in later stages of human immunodeficiency virus (HIV) infection, evidence has been provided for a specific unresponsiveness to triggering either by specific antigen in the context of autologous major histocompatibility molecules (self + X) or anti-CD3 monoclonal antibodies (MAb) in both CD4 and CD8 cells from asymptomatic HIV-infected individuals. In the present study we analyzed this unresponsiveness using mitogenic antibodies to distinct T cell membrane receptors. T cells from HIV-infected men who had normal numbers of CD4+ T cells responded poorly to activation signals via the CD3 membrane antigen in both accessory cell-dependent as well as accessory cell-independent culture systems. A similar low response was observed in an anti-CD2-driven system. In contrast, proliferation induced by anti-CD3, anti-CD2, or the phorbol ester Phorbol myristate acetate could be normally enhanced by anti-CD28 MAb. We demonstrated that this unresponsiveness is not due to a failure to induce early events required for activation, such as increased intracellular concentration of free calcium and activation of protein kinase C, but is caused by an imbalance between naive and memory T cells. In HIV-infected asymptomatic men, CD29+ memory T cells are selectively depleted which results in a poor responsiveness to self + X. These findings provide new insights that may have implications for our understanding of the immunopathogenesis of AIDS.

Structural heterogeneity of a human melanoma-associated antigen.

The biosynthesis, structure, and topology of a melanoma-associated antigen, previously defined with the monoclonal antibody NKI/C-3 was studied. A polyclonal rabbit antiserum was raised against the antigen with a broader reactivity than the previously used monoclonal antibody NKI/C-3. The antigen was shown to consist of a single protein backbone to which two or three N-linked glycans were added cotranslationally. Extensive further heterogeneity was generated in the Golgi compartment and was shown to be dependent on the presence of complex type sugars. Although the antigen is associated with melanomas, it was not codistributed with the tyrosinase activity associated with melanogenesis. The antigen did show codistribution with cathepsin D, which is a marker for lysosomal functions.

HIV-2 infection in 12 European residents: virus characteristics and disease progression.

OBJECTIVE: To assess the disease progression rate among 12 HIV-2-infected West European residents (nine of West African descent), compared with the disease progression rate among HIV-1-infected individuals of the same population, and the characteristics of the HIV-2 strains involved. METHODS: HIV-2-infected individuals were identified by commercially available serological assays, their clinical status and CD4+ cell counts were monitored, and HIV-2 was isolated from their peripheral blood mononuclear cells. T-cell-line tropism and syncytium-inducing capacities of the isolated viruses were determined and their phylogenetic relationships were analysed by comparing polymerase chain reaction-amplified nucleotide sequences of reverse transcriptase (RT) gene segments. RESULTS: Eight of the 12 HIV-2-infected individuals presented with progressive disease and one of them progressed from Centers for Disease Control and Prevention group A1 to A3 within 36 months after seroconversion. The ratios of asymptomatic versus symptomatic individuals among residents of the Rotterdam region of West African descent were 2:7 for HIV-2 and 8:9 for HIV-1-infected individuals. HIV-2 was isolated from six of the nine individuals with progressive disease. The time required for virus isolation correlated inversely with the individuals' CD4+ cell counts. Five of the HIV-2 isolates replicated in immortalized T-cell lines, and two isolates from patients with AIDS were syncytium-inducing. Five HIV-2 isolates from patients born in the Cape Verdian Isles grouped together within subtype A. The HIV-2 isolate from a patient of Ghanese origin belonged to subtype B. Mutations were identified in the RT genes from HIV-2 isolates of two zidovudine-treated patients, one of which has also been shown to be involved in zidovudine resistance in HIV-1. CONCLUSION: Disease progression in HIV-2 infection may be as rapid as in HIV-1. HIV-2 isolation and viral phenotype were related to disease status, and mutations identical to those observed in HIV-1 zidovudine resistance were observed in patients treated with zidovudine.

Differences in clinical course in zidovudine-treated asymptomatic HIV-infected men associated with T-cell function at intake.

Declining CD4+ T-cell numbers and anti-CD3-induced T-cell responsiveness are prognostic markers for progression of HIV infection. We investigated the effect of long-term (2-year) zidovudine treatment on these immunological markers in a group of nine asymptomatic p24-antigenaemic men, five of whom progressed to AIDS. A group of 10 untreated HIV-infected men, five of whom progressed to AIDS, was studied as a control. At intake, 1 year before the start of treatment, CD4+ T-cell numbers in the groups were not significantly different. However, at that time progressors already exhibited an extremely low anti-CD3-induced T-cell responsiveness compared with non-progressors. In all people T-cell responsiveness and the number of CD4+ T-cells had improved 6 months after the start of zidovudine treatment. However, CD4+ T-cell numbers were not persistently elevated, and restoration of T-cell responsiveness was of only short duration. Our results show that zidovudine treatment in the asymptomatic phase of HIV infection did not result in a sustained improvement in T-cell function. Furthermore, they suggest that differences in clinical course among zidovudine-treated asymptomatics may be caused by heterogeneity of this group with respect to T-cell functional capacity at the start of treatment.

Immunological and virological markers in individuals progressing from seroconversion to AIDS.

Six men were selected from a large cohort of homosexual men participating in a study on HIV infection that was followed from seroconversion to AIDS. The patients were studied retrospectively for immunological functions of T cells, T-cell subset distribution and biological phenotype of HIV. A severe decrease in anti-CD3 monoclonal antibody (MAb)-induced T-cell proliferation at seroconversion was observed in two out of six men. After this acute phase, CD4+ T-cell numbers were in the normal range in the early asymptomatic period; the proliferative response was subnormal, whereas the capacity to generate cytotoxic T cells (CTL) was normal. From seroconversion on, CD4+CD29+ memory T-cell numbers were decreased to approximately 50% of normal values, which may contribute to loss of T-cell reactivity. In the asymptomatic phase only slow-replicating non-syncytium-inducing HIV variants were observed. The T-cell proliferative response further declined with the depletion of naive CD4+ CD45RA+ T cells and CD4+ T-cell numbers started to decline. This second decrease in T-cell function coincided with the emergence of more rapidly replicating, often (four out of six) syncytium-inducing variants. At diagnosis of AIDS, T-cell proliferation and CD4+ T-cell numbers were extremely low in five out of six patients and CTL function had declined in three out of five individuals tested. Circulating CD8+ cells had gradually shifted to an immature CD38+CD28- phenotype. Our findings support the theory that HIV-induced immune dysfunction allows for the emergence of virulent HIV variants associated with CD4+ cell loss and disease.

Early replication steps but not cell type-specific signalling of the viral long terminal repeat determine HIV-1 monocytotropism.

The expression of human immunodeficiency virus type 1 (HIV-1) is enhanced after cell activation because of the interaction of cell-encoded nuclear factors that interact with binding sites in the long terminal repeats (LTRs). Here we studied the contribution of cell type-specific activation signals to differences in cytotropism of HIV-1 variants. Four closely related molecular HIV-1 clones with distinct biological phenotypes and different capacities to replicate in primary monocyte-derived macrophages (MDMs) or T cell lines were used. Sequence analysis of these LTRs revealed variation in functionally important regions. Adaptation of virus variants to particular host cells by differences in LTR responsiveness was analyzed. LTR-CAT constructs were transiently transfected in T cells that were stimulated with T cell-specific activation signals such as combinations of anti-CD3 or anti-CD28 MoAB or in primary monocytes that were stimulated with IL-3, IL-4, or GM-CSF. No differences in responsiveness to cell type-specific signals were demonstrated. To further elucidate the level of restriction in cell tropism, transfection of four full-length infectious molecular HIV-1 clones into 5-day cultured MDMs was performed. From all clones, competent virus could be rescued from MDMs by coculture with PHA-stimulated PBLs. However, following cell-free inoculation, proviral DNA could be detected by PCR analysis only in monocytes exposed to HIV-1 clones that previously were shown to establish productive infection.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular characterization of HIV viruses generated after in vivo ligation.

During the course of infection, human immunodeficiency virus type 1 (HIV-1) displays wide genotypic and phenotypic differences. Construction of chimeric viruses is useful to determine the genotypic basis that underlies phenotypic variations, but the procedure is time-consuming. Previously, it has been shown that co-transfection of truncated hemi-genomic HIV-1 proviral DNA can lead to generation of full-length infectious virus. In the study of HIV phenotypes, using this technique, it is important to determine whether recombination between the two hemigenomes occurs without mutations. After co-transfection, progeny recombinant viruses replicated at the same rate as the control. We purified progeny viruses from culture supernatants and determined mutations at the recombination site. It appeared that correct in vivo ligation depended on the purity of DNA and the restriction site used. It also appeared that some of the mutations observed affect replication, as progeny viruses bearing one of these mutations disappeared during in vitro cultures, whereas other mutants did not. Although this technique is widely applied to generate chimeric viruses, the results should be evaluated with care, since mutations influencing the phenotype of the progeny viruses may have been introduced.

Development of a real-time quantitative RT-PCR for the detection of HIV-2 RNA in plasma.

An assay is described for the quantification of human immunodeficiency virus type 2 (HIV-2) RNA in EDTA plasma based on RT-PCR using the Taqman real-time PCR detection method. As standard, an electron microscopically counted virus stock of HIV-2 strain NIHZ was used. The lower detection limit is 5 # 102 HIV-2 RNA copies per ml of EDTA plasma. The assay is linear within the range required (5 # 102-106 HIV-2 RNA copies/ml of EDTA plasma) with an intra assay variability of 2.5% and an inter-assay variability ranging from 2% at 106 copies to 7.5% at the lower detection limit. Three primer/probe combinations were developed to circumvent false negative samples due to nucleotide variation in the target sequence. Using these primer/probe sets enabled the detection of HIV-2 DNA sequences from all HIV-2 seropositive individuals and two out of five dual human immunodeficiency virus type 1 (HIV-1) and HIV-2 seropositive individuals visiting the University Hospital Rotterdam.

Vaccine-induced enhancement of viral infections.

Examples of vaccine-induced enhancement of susceptibility to virus infection or of aberrant viral pathogenesis have been documented for infections by members of different virus families. Several mechanisms, many of which still are poorly understood, are at the basis of this phenomenon. Vaccine development for lentivirus infections in general, and for HIV/AIDS in particular, has been little successful. Certain experimental lentiviral vaccines even proved to be counterproductive: they rendered vaccinated subjects more susceptible to infection rather than protecting them. For vaccine-induced enhanced susceptibility to infection with certain viruses like feline coronavirus, Dengue virus, and feline immunodeficiency virus, it has been shown that antibody-dependent enhancement (ADE) plays an important role. Other mechanisms may, either in the absence of or in combination with ADE, be involved. Consequently, vaccine-induced enhancement has been a major stumble block in the development of certain flavi-, corona-, paramyxo-, and lentivirus vaccines. Also recent failures in the development of a vaccine against HIV may at least in part be attributed to induction of enhanced susceptibility to infection. There may well be a delicate balance between the induction of protective immunity on the one hand and the induction of enhanced susceptibility on the other. The present paper reviews the currently known mechanisms of vaccine-induced enhancement of susceptibility to virus infection or of aberrant viral pathogenesis.

The loss of immunodominant epitopes affects interferon-gamma production and lytic activity of the human influenza virus-specific cytotoxic T lymphocyte response in vitro.

In the present study, we examined the effect of the loss of the human leucocyte antigen (HLA)-B*3501-restricted nucleoprotein (NP)(418-426) epitope on interferon (IFN)-gamma-production and lytic activity of the human cytotoxic T lymphocyte (CTL) response in vitro. Extensive amino acid variation at T cell receptor contact residues of the NP(418-426) epitope has led to repeated evasion from specific CTL. We generated recombinant influenza viruses with variants of the NP(418-426) epitope, which were used to stimulate peripheral blood mononuclear cells obtained from six HLA-B*3501-positive study subjects in order to expand virus-specific CTL. Loss of the NP(418-426) epitope resulted in a significant reduction of IFN-gamma-expressing CD8+ T cells, similar to that observed previously after the loss of the HLA-B*2705-restricted NP(383-391) epitope. In addition, the effect of the loss of the NP(418-426) epitope on the lytic activity of the virus-specific CTL response was assessed. Also this functional property of the virus-specific CTL response was affected significantly by the loss of this and the NP(383-391) epitope, as determined using the newly developed fluorescent antigen-transfected target cell (FATT)-CTL assay. These findings indicate that the loss of single immunodominant epitopes affects the functionality of the virus-specific CTL response significantly.

CCR5, GPR15, and CXCR6 are major coreceptors of human immunodeficiency virus type 2 variants isolated from individuals with and without plasma viremia.

Human immunodeficiency virus type 2 (HIV-2) is generally considered capable of using a broad range of coreceptors. Since HIV-2 variants from individuals with nonprogressive infection were not studied previously, the possibility that broad coreceptor usage is a property of variants associated with progressive infection could not be excluded. To test this, we determined the coreceptor usage of 43 HIV-2 variants isolated from six long-term-infected individuals with undetectable plasma viremia. Using GHOST indicator cells, we showed for the first time that the only coreceptors efficiently used by low-pathogenic HIV-2 variants are CCR5, GPR15 (BOB), and CXCR6 (BONZO). Surprisingly, control HIV-2 variants (n = 45) isolated from seven viremic individuals also mainly used these three coreceptors, whereas use of CCR1, CCR2b, or CCR3 was rare. Nearly a quarter of all HIV-2 variants tested could infect the parental GHOST cells, which could be partially explained by CXCR4 usage. Use of CXCR4 was observed only for HIV-2 variants from viremic individuals. Thirty-eight variants from aviremic and viremic HIV-2-infected individuals were additionally tested in U87 cells. All except one were capable of infecting the parental U87 cells, often with high efficiency. When virus production in parental cells was regarded as background in the coreceptor-transduced cell lines, the results in U87 cells were largely in agreement with the findings in GHOST cells. HIV-2 isolates from aviremic individuals commonly use as coreceptors CCR5, GPR15, and CXCR6, as well as an unidentified receptor expressed by U87 cells. Broad coreceptor usage, therefore, does not appear to be associated with pathogenicity of HIV-2.

Coreceptor usage of human immunodeficiency virus type 2 primary isolates and biological clones is broad and does not correlate with their syncytium-inducing capacities.

Entry of human immunodeficiency virus type 1 (HIV-1) into target cells is mediated by binding of the surface envelope glycoprotein to the CD4 molecule. Interaction of the resulting CD4-glycoprotein complex with alpha- or beta-chemokine receptors, depending on the biological phenotype of the virus, then initiates the fusion process. Here, we show that primary HIV-2 isolates and biological clones, in contrast to those of HIV-1, may use a broad range of coreceptors, including CCR-1, CCR-3, CCR-5, and CXCR-4. The syncytium-inducing capacity of these viruses did not correlate with the ability to infect via CXCR-4 or any other coreceptor. One cell-free passage of the intermediate isolates in mitogen-stimulated, CD8+ cell-depleted peripheral blood mononuclear cells resulted in the outgrowth of variants with CCR-5 only, whereas the coreceptor usage of late and early isolates did not change. Since HIV-2 is less pathogenic in vivo than HIV-1, these data suggest that HIV pathogenicity in vivo is not directly related to the spectrum of coreceptors used in in vitro systems.

Kinetics of antiviral activity by human immunodeficiency virus type 1-specific cytotoxic T lymphocytes (CTL) and rapid selection of CTL escape virus in vitro.

The antiviral activity of a CD8(+) cytotoxic T-lymphocyte (CTL) clone (TCC108) directed against a newly identified HLA-B14-restricted epitope, human immunodeficiency virus type 1 (HIV-1) Rev(67-75) SAEPVPLQL, was analyzed with respect to its kinetics of target cell lysis and inhibition of HIV-1 production. Addition of TCC108 cells or CD8(+) reverse transcriptase-specific CTLs to HLA-matched CD4(+) T cells at different times after infection with HIV-1 IIIB showed that infected cells became susceptible to CTL-mediated lysis before peak virus production but after the onset of progeny virus release. When either of these CTLs were added to part of the infected cells immediately after infection, p55 expression and virus production were significantly suppressed. These data support a model in which CTLs, apart from exerting cytolytic activity which may prevent continued virus release, can interfere with viral protein expression during the eclipse phase via noncytolytic mechanisms. TCC108-mediated inhibition of virus replication in peripheral blood mononuclear cells caused rapid selection of a virus with a mutation (69E-->K) in the Rev(67-75) CTL epitope which abolished recognition by TCC108 cells. Taken together, these data suggest that both cytolytic and noncytolytic antiviral mechanisms of CTLs can be specifically targeted to HIV-1-infected cells.

Selective loss of T cell functions in different stages of HIV infection. Early loss of anti-CD3-induced T cell proliferation followed by decreased anti-CD3-induced cytotoxic T lymphocyte generation in AIDS-related complex and AIDS.

To investigate the effects of persistant human immunodeficiency virus (HIV) infection on T cell reactivity, functional properties of peripheral blood T cells from HIV-seropositive homosexual men in various stages of infection were studied. T cell activation via CD3 resulting in proliferation and differentiation was measured in a model system independent of accessory cells, using immobilized anti-CD3 monoclonal antibodies (mAb). T cells from HIV-infected asymptomatic men had a decreased proliferative response compared to HIV-negative controls. T cells from AIDS-related complex (ARC) and AIDS patients, compared to T cells from asymptomatic HIV-infected men, had a significantly lower proliferative response to anti-CD3 mAb. This diminished response to anti-CD3 mAb was shown to be due to decreased interleukin (IL) 2 production and could be enhanced by co-stimulation with anti-CD28 mAb or by adding IL 2. Anti-CD3-induced generation of cytotoxic T lymphocytes was fully intact in early infection but was severely decreased in T cells from ARC and AIDS patients. Cytotoxic activity could be restored to near normal levels after co-stimulation with either anti-CD28 mAb or IL 2. Our data demonstrate a differential loss of T cell functions in the course of HIV infection which is predominantly caused by a lack of IL 2 production after stimulation via the CD3/T cell receptor complex. In early HIV infection this seems to be predominantly caused by a specific loss of memory T cells. However, in later stages of infection when both naive and memory T cell subsets are depleted, resulting in a normal naive/memory T cell ratio, T cell functions further deteriorate probably due to intrinsic activation defects. These findings may be of pathogenic relevance since diminished T cell reactivity may facilitate spreading and replication of virulent HIV variants heralding development of ARC and AIDS.

Differential syncytium-inducing capacity of human immunodeficiency virus isolates: frequent detection of syncytium-inducing isolates in patients with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex.

Human immunodeficiency virus isolates were studied with respect to syncytium-inducing capacity, replicative properties, and host range. Five of 10 isolates from patients with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex were able to induce syncytia in cultures of peripheral blood mononuclear cells (MNC). In contrast, only 2 of 12 isolates from asymptomatic individuals had syncytium-inducing capacity. Syncytium-inducing isolates were reproducibly obtained from the same MNC sample in over 90% of the cases, independent of the donor MNC used for propagation. Syncytium-inducing capacity was shown to be a stable property of an isolate, independent of viral replication rates. Evidence was obtained that the high replication rate of syncytium-inducing isolates observed during primary isolation may be due to higher infectivity of these isolates. The finding that only syncytium-inducing isolates could be transmitted to the H9 cell line is compatible with this higher infectivity. The frequent isolation of syncytium-inducing isolates from individuals with AIDS-related complex or AIDS and the apparent higher in vitro infectivity of these isolates suggest that syncytium-inducing isolates may unfavorably influence the course of human immunodeficiency virus infection.

Siderophores and outer membrane proteins of antagonistic, plant-growth-stimulating, root-colonizing Pseudomonas spp.

As an approach to understanding the molecular basis of the reduction in plant yield depression by root-colonizing Pseudomonas spp. and especially of the role of the bacterial cell surfaces in this process, we characterized 30 plant-root-colonizing Pseudomonas spp. with respect to siderophore production, antagonistic activity, plasmid content, and sodium dodecyl sulphate-polyacrylamide gel electrophoresis patterns of their cell envelope proteins. The results showed that all strains produce hydroxamate-type siderophores which, because of the correlation with Fe3+ limitation, are thought to be the major factor responsible for antagonistic activity. Siderophore-negative mutants of two strains had a strongly decreased antagonistic activity. Five strains maintained their antagonistic activity under conditions of iron excess. Analysis of cell envelope protein patterns of cells grown in excess Fe3+ showed that most strains differed from each other, although two classes of similar or identical strains were found. In one case such a class was subdivided on the basis of the patterns of proteins derepressed by iron limitation. Small plasmids were not detected in any of the strains, and only one of the four tested strains contained a large plasmid. Therefore, it is unlikely that the Fe3+ uptake system of the antagonistic strains is usually plasmid encoded.