Attraction of mosquitoes to primate odours and implications for zoonotic Plasmodium transmission.

Vector-borne diseases often originate from wildlife and can spill over into the human population. One of the most important determinants of vector-borne disease transmission is the host preference of mosquitoes. Mosquitoes with a specialised host preference are guided by body odours to find their hosts in addition to carbon dioxide. Little is known about the role of mosquito host preference in the spillover of pathogenic agents from humans towards animals and vice versa. In the Republic of Congo, the attraction of mosquitoes to primate host odours was determined, as well as their possible role as malaria vectors, using odour-baited traps mimicking the potential hosts of mosquitoes. Most of the mosquito species caught showed a generalistic host preference. Anopheles obscurus was the most abundant Anopheles mosquito, with a generalistic host preference observed from the olfactory response and the detection of various Plasmodium parasites. Interestingly, Culex decens showed a much higher attraction towards chimpanzee odours than to human or cow odours. Human Plasmodium parasites were observed in both human and chimpanzee blood, although not in the Anopheles mosquitoes that were collected. Understanding the role of mosquito host preference for cross-species parasite transmission provides information that will help to determine the risk of spillover of vector-borne diseases.

Oral administration of different forms of a tolerogenic peptide to define the preparations and doses that delay anti-DNA antibody production and nephritis and prolong survival in SLE-prone mice.

Therapeutic agents currently in use to treat systemic lupus erythematosus (SLE) are predominantly immunosuppressive agents with limited specificities. Multiple groups, including ours, have illustrated that inducing tolerance in SLE animal models ameliorates disease symptoms and increases survival. We examined if oral administration of a tolerogenic peptide could affect SLE disease progression. The pConsensus (pCons) peptide, based on protein sequences of anti-double stranded (anti-ds)DNA antibodies, induces tolerance through upregulation of regulatory T cells when administered intravenously. Six different forms of pCons, including multiple antigenic peptides (MAP) and cyclic peptides made up of L- and D-amino acids, at three different concentrations, were fed to BWF1 SLE-susceptible mice for 30 weeks. Mice fed 100 µg of L-MAP or D-MAP had less cumulative proteinuria and serum anti-dsDNA antibody levels than controls. In addition, animals in these groups also survived significantly longer than controls with a corresponding increase in serum transforming growth factor beta (TGFß, implying a protective role for pCons-induced regulatory T cells. Oral administration of a tolerogenic peptide is a safe, effective method for ameliorating SLE disease manifestations and prolonging survival in SLE-prone mice. Induction of oral tolerance using modified pCons peptides could lead to a novel targeted therapy for human SLE.

Galectin-1-induced down-regulation of T lymphocyte activation protects (NZB x NZW) F1 mice from lupus-like disease.

Systemic lupus erythematosus (SLE) is a complex autoimmune disease characterized by a hyperactive immune system, including activation of autoreactive T and B cells. These studies demonstrate that administration of recombinant galectin-1, a ß-galactose binding protein, to SLE-prone (NZB × NZW) F1 mice reduced lymphocyte activation, inhibited serum anti-double-stranded DNA(dsDNA) IgG antibody production, decreased the incidence of proteinuria, and increased survival rate. In addition, recombinant galectin-1'-treated mice had a higher frequency of Foxp3 expression, which suggested an increase in the percentage of peripheral regulatory T cells. Consistent with the finding that there were fewer activated T lymphocytes, ex vivo T cells from mice treated with recombinant galectin-1 exhibited less proliferation in response to TCR stimulation. Furthermore, these cells were less efficient at lipid raft clustering in response to TCR/CD28 engagement, consistent with published reports that galectin-1 can reorganize the synaptic contact to interfere with TCR signaling and activation to prevent T cell activation. Aged galectin-1-deficient mice had higher serum levels of antibodies against dsDNA, elucidating a role for endogenous galectin-1 in decreasing susceptibility to autoimmunity. Together, the findings highlight galectin-1 as a novel potential therapeutic immune modulator for treatment of lupus-like disease.

Interferon-inducible gene 202b controls CD8(+) T cell-mediated suppression in anti-DNA Ig peptide-treated (NZB × NZW) F1 lupus mice.

Administration of an artificial peptide (pConsensus) based on anti-DNA IgG sequences that contain major histocompatibility complex class I and class II T-cell determinants, induces immune tolerance in NZB/NZW F1 female (BWF1) mice. To understand the molecular basis of CD8(+) Ti-mediated suppression, we previously performed microarray analysis to identify genes that were differentially expressed following tolerance induction with pCons. CD8(+) T cells from mice tolerized with pCons showed more than two-fold increase in Ifi202b mRNA, an interferon inducible gene, versus cells from untolerized mice. Ifi202b expression increased through weeks 1-4 after tolerization and then decreased, reapproaching baseline levels at 6 weeks. In vitro polyclonal activation of tolerized CD8(+) T cells significantly increased Ifi202b mRNA expression. Importantly, silencing of Ifi202b abrogated the suppressive capacity of CD8(+) Ti cells. This was associated with decreased expression of Foxp3, and decreased gene and protein expression of transforming growth factor (TGF)ß and interleukin-2 (IL-2), but not of interferon (IFN)-γ, IL-10, or IL-17. Silencing of another IFN-induced gene upregulated in tolerized CD8(+) T cells, IFNAR1, had no effect on the ability of CD8(+) T cells to suppress autoantibody production. Our findings indicate a potential role for Ifi202b in the suppressive capacity of peptide-induced regulatory CD8(+) Ti cells through effects on the expression of Foxp3 and the synthesis of TGFß.

Pro-inflammatory high-density lipoproteins and atherosclerosis are induced in lupus-prone mice by a high-fat diet and leptin.

Atherosclerosis is accelerated in people with systemic lupus erythematosus, and the presence of dysfunctional, pro-inflammatory high-density lipoproteins is a marker of increased risk. We developed a mouse model of multigenic lupus exposed to environmental factors known to accelerate atherosclerosis in humans - high-fat diet with or without injections of the adipokine leptin. BWF1 mice were the lupus-prone model; BALB/c were non-autoimmune controls. High-fat diet increased total serum cholesterol in both strains. In BALB/c mice, non-high-density lipoprotein cholesterol levels increased; they did not develop atherosclerosis. In contrast, BWF1 mice on high-fat diets developed increased quantities of high-density lipoproteins as well as elevated high-density lipoprotein scores, indicating pro-inflammatory high-density lipoproteins; they also developed atherosclerosis. In the lupus-prone strain, addition of leptin increased pro-inflammatory high-density lipoprotein scores and atherosclerosis, and accelerated proteinuria. These data suggest that environmental factors associated with obesity and metabolic syndrome can accelerate atherosclerosis and disease in a lupus-prone background.

Some antiphospholipid antibodies bind to various serine proteases in hemostasis and tip the balance toward hypercoagulant states.

The body has an elaborate system that maintains blood circulation and rapidly stops bleeding when vessels are damaged. Abnormalities that disrupt this balance may lead to thrombosis. While beta(2)-glycoprotein I is generally accepted as the major antigen for antiphospholipid antibodies in the antiphospholipid syndrome, our accumulated studies show that some antiphospholipid antibodies bind homologous enzymatic domains of several serine proteases involved in hemostasis and fibrinolysis. Functionally, some of the protease-reactive antiphospholipid antibodies hinder anticoagulant regulation and resolution of clots, thus tip the balance toward thrombosis. Intriguingly, several serine protease-reactive antiphospholipid antibodies also react with beta(2)-glycoprotein I, and interactions between antiphospholipid antibodies and antigens are cross-inhibited, indicating that these antiphospholipid antibodies recognize conformational epitope(s) on beta(2)-glycoprotein I and target serine proteases. Viewed as a whole, these results extend previous reports that antiphospholipid antibodies bind to various hemostasis factors, and provide a new perspective about some antiphospholipid antibodies in terms of their binding specificities and related functional properties in promoting thrombosis.

Distinct gene signature revealed in white blood cells, CD4(+) and CD8(+) T cells in (NZBx NZW) F1 lupus mice after tolerization with anti-DNA Ig peptide.

Tolerizing mice polygenically predisposed to lupus-like disease (NZB/NZW F1 females) with a peptide mimicking anti-DNA IgG sequences containing MHC class I and class II T cell determinants (pConsensus, pCons) results in protection from full-blown disease attributable in part to the induction of CD4(+)CD25(+)Foxp3+ and CD8(+)Foxp3+ regulatory T cells. We compared 45 000 murine genes in total white blood cells (WBC), CD4(+) T cells, and CD8(+) T cells from splenocytes of (NZBxNZW) F1 lupus-prone mice tolerized with pCons vs untreated naïve mice and found two-fold or greater differential expression for 448 WBC, 174 CD4, and 60 CD8 genes. We identified differentially expressed genes that played roles in the immune response and apoptosis. Using real-time PCR, we validated differential expression of selected genes (IFI202B, Bcl2, Foxp3, Trp-53, CCR7 and IFNar1) in the CD8(+)T cell microarray and determined expression of selected highly upregulated genes in different immune cell subsets. We also determined Smads expression in different immune cell subsets, including CD4(+) T cells and CD8(+) T cells, to detect the effects of TGF-beta, known to be the major cytokine that accounts for the suppressive capacity of CD8(+) Treg in this system. Silencing of anti-apoptotic gene Bcl2 or interferon genes (IFI202b and IFNar1 in combination) in CD8(+) T cells from tolerized mice did not affect the expression of the other selected genes. However, silencing of Foxp3 reduced expression of Foxp3, Ifi202b and PD1-all of which are involved in the suppressive capacity of CD8(+) Treg in this model.

Quantitating the multiplicity of infection with human immunodeficiency virus type 1 subtype C reveals a non-poisson distribution of transmitted variants.

Identifying the specific genetic characteristics of successfully transmitted variants may prove central to the development of effective vaccine and microbicide interventions. Although human immunodeficiency virus transmission is associated with a population bottleneck, the extent to which different factors influence the diversity of transmitted viruses is unclear. We estimate here the number of transmitted variants in 69 heterosexual men and women with primary subtype C infections. From 1,505 env sequences obtained using a single genome amplification approach we show that 78% of infections involved single variant transmission and 22% involved multiple variant transmissions (median of 3). We found evidence for mutations selected for cytotoxic-T-lymphocyte or antibody escape and a high prevalence of recombination in individuals infected with multiple variants representing another potential escape pathway in these individuals. In a combined analysis of 171 subtype B and C transmission events, we found that infection with more than one variant does not follow a Poisson distribution, indicating that transmission of individual virions cannot be seen as independent events, each occurring with low probability. While most transmissions resulted from a single infectious unit, multiple variant transmissions represent a significant fraction of transmission events, suggesting that there may be important mechanistic differences between these groups that are not yet understood.

Atherosclerosis and systemic lupus erythematosus: the role of altered lipids and of autoantibodies.

The accelerated atherosclerosis that occurs in some patients with systemic lupus erythematosus (SLE) has a complex pathogenesis, including alterations in lipids, inflammation and the immune system. In this article, we review the evidence that peroxidase-related alteration of normal, protective high-density lipoprotein (HDL) converts them to pro-inflammatory HDL (piHDL), characterized by lower content of the cholesterol transport lipoprotein ApoA1 and impaired function of the antioxidant enzyme paroxonase, which prevents oxidation of low-density lipoprotein (LDL). Forty-five per cent of women with SLE have piHDL compared with 20% of patients with rheumatoid arthritis and 4% of healthy controls. The presence of piHDL increases risk for coronary artery events and carotid artery plaque. Another result of lipid oxidation in patients with SLE is generation of highly oxidized LDL and phospholipids (PL), probably stimulating antibodies to OxPL phospholipids. These antibodies along with promoting thrombosis also interfere with deposits of Annexin V onto endothelial cells, which probably promote increased instability of atherosclerotic plaque. Thus, piHDL and anti-OxPL promote plaque formation, plaque instability and thrombosis, accounting for some of the large increase in atherosclerosis and coronary artery events in SLE.

Assessment of American College of Rheumatology quality criteria for rheumatoid arthritis in a pre-quality criteria patient cohort.

OBJECTIVE: To evaluate the American College of Rheumatology (ACR) starter set of quality measures for rheumatoid arthritis (RA) in an actual patient cohort that preceded publication of the quality measures. METHODS: We retrospectively applied the 2006 ACR quality criteria to a prospectively studied cohort of 568 patients with RA treated by 1,932 unique physicians including 255 different rheumatologists between the years 1999 and 2003. Data on performance were obtained from self-report surveys and medical record review within 12 months. RESULTS: At least 1 joint examination was performed in 98% of patients. Patient and physician global assessments were reported for 79% and 74% of patients, respectively. A total of 85% of patients received disease-modifying antirheumatic drugs (DMARDs). DMARD adjustments were made for 50% of patients in whom increasing disease activity was noted at least once and for 64% of patients in whom increasing disease activity was noted during 2 (of 4) 3-month periods within the year. Compared with self-report surveys, medical records substantially underreported performance on quality measures. CONCLUSION: The ACR-endorsed quality measures for RA can be assessed using available data sources. When both self-report and medical record data are used, adherence rates, designed to serve as minimum standards of care, were moderate or high for most measures. Prior to using indicators to compare quality across groups, specific strategies for operationalizing measures and for using accurate data sources to assess adherence to the measures should be defined.

Application of explicit process of care measurement to rheumatoid arthritis: Moving from evidence to practice.

OBJECTIVE: To construct quality measures with measurement validity and meaning for clinicians. METHODS: We conducted a prospective cohort study of rates of change in disease-modifying antirheumatic drug (DMARD) and/or systemic corticosteroid drug or dose for 568 patients with rheumatoid arthritis (RA) across 6,159 clinical encounters within 12 months to examine how changes in clinical specifications change adherence. RESULTS: Rates of DMARD change were sensitive to specifications regarding the intensity of disease activity (severe or moderate), duration of specified disease activity, and length of the observation period. Over 12 months, the proportions of 377 patients with severe disease activity observed for 1-month, 2-month, and 3-month time blocks who had a change in DMARD drug or dose were 36%, 57%, and 74%, respectively. Over 12 months, a change in DMARD drug or dose was observed for 44%, 50%, and 68% of 377 patients with severe disease within 3 months, 6 months, and 12 months, respectively, of the patient meeting criteria for severe disease activity. A change in DMARD drug or dose was observed for 21%, 23%, and 34% of 149 patients with moderate disease activity within 3, 6, and 12 months, respectively, of the patient meeting criteria for moderate disease activity. CONCLUSION: Rates of pharmacologic interventions for patients with moderate and severe RA disease activity vary substantially by intensity and duration of disease activity and by duration of period for observing change. Lack of precision in explicit process criteria could substantially mislead comparisons of quality of care across comparison groups.

Peptides from antibodies to DNA elicit cytokine release from peripheral blood mononuclear cells of patients with systemic lupus erythematosus: relation of cytokine pattern to disease duration.

Peptides from VH regions of antibodies to DNA drive immune responses in systemic lupus erythematosus (SLE). We studied peptide-induced cytokine release by peripheral blood mononuclear cells (PBMC) of patients, the influence of peptide concentration, disease characteristics and HLA-D haplotypes. Cells secreting cytokines (IFNgamma, IL-2, IL-4 and IL-10) were measured by ELISPOT in PBMC from 31 patients with SLE and 20 matched healthy controls in response to seven peptides (A-G) from the CDR1/FR2 to CDR2/FR3 VH regions of human anti-DNA MAbs. Disease activity was assessed by SELENA-SLEDAI. HLA-DR and -DQ alleles were determined by molecular typing techniques. PBMC from significantly higher proportions of SLE patients than controls responded to VH peptides by generating IFNgamma and IL-10. Type of cytokines released in response to at least one peptide (D) depended on antigen concentration. Cytokine release was not associated with clinical features of SLE except for disease duration. A shift occurred from IFNgamma, IL-4 and IL-10 production in early disease to IL-4 and IL-10 in late disease (suggesting increasing TH2-like responses over time). Three peptides (B, D, G) were more stimulatory in the SLE patients than controls. Although none of the peptides was restricted by any particular MHC class II allele, among responders there was increased prevalence of HLA- DQB1*0201 and/or DRB1*0301, alleles known to predispose to SLE. Thus, responses to some VH peptides are more frequent in SLE and vary with disease duration. Increased responses in individuals with HLA class II genotypes that predispose to SLE suggest that peptide presentation by those molecules permits brisker peripheral blood cell responses to autoantibody peptides, thus increasing risk for disease.

Protein interaction for an interferon-inducible systemic lupus associated gene, IFIT1.

OBJECTIVE: To identify disease-related genes and immune-regulatory pathways in the pathogenesis of systemic lupus erythematosus (SLE) by using gene expression profiling and protein-protein interaction analysis. METHODS: Peripheral white blood cell gene expression profiles of 10 SLE patients were determined by oligonucleotide microarray analysis. Clustering of the gene expression profile was compared with the clinical immune phenotype. SLE-induced genes that were over- or under-expressed were determined and independently validated using a real-time polymerase chain reaction (PCR) method. To study their potential function and the possible pathways involved, a candidate gene was cloned and a GST (glutathione S-transferase) fusion protein was expressed in Escherichia coli. The fusion protein was further purified using the glutathione Sepharose 4B system, and was treated as bait to capture prey from SLE peripheral white blood cell lysate. MALDI-TOF (matrix-assisted laser desorption/ionization-time-of-flight) mass spectrometry was then performed to determine the prey protein. RESULTS: Similarity was found between the gene expression profile and the immune phenotype clusters of the SLE patients. More than 20 disease-associated genes were identified, some of which have not been related to SLE previously. Of these genes, a cluster of interferon-induced genes were highly correlated. IFIT1 (interferon-induced with tetratricopeptide repeats 1) was one of these genes, and overexpression of its mRNA was confirmed independently by real-time PCR in a larger population (40 SLE patients and 29 normal controls). An IFIT1 protein- protein interaction study showed that IFIT1 may interact with Rho/Rac guanine nucleotide exchange factor. CONCLUSION: The gene expression profile seems to be the molecular basis of the diverse immune phenotype of SLE. On the basis of the SLE-related genes found in this study, we suggest that the interferon-related immune pathway is important in the pathogenesis of SLE. IFIT1 is the first gene described as a candidate gene for SLE, and may function by activating Rho proteins through interaction with Rho/Rac guanine nucleotide exchange factor. IFIT1 and the interferon-related pathway may provide potential targets for novel interventions in the treatment of SLE.

The role of VH determinants in systemic lupus erythematosus.

The V-regions of anti-DNA antibodies contain determinants which can drive the autoimmune in SLE. Most of the evidence comes from murine studies where VH-derived epitopes accelerate the disease process in lupus prone-mice and can elicit mild inflammatory changes reminiscent of lupus in healthy animals. T helper cells reactive with VH peptides arise spontaneously during the disease and are thought to assist production of both anti-peptide antibodies and the generation of autoantibodies that deposit in the glomeruli. In mice stimulatory epitopes may be unique to autoantibodies. As tolerogens VH peptides may delay or diminish the autoimmune response by altering the production of cytokines. An artificial VH peptide, (pCONCENSUS) has been derived and this inhibits responses to VH and other autoantigens but leaves the murine immune system intact and able to generate reponses to external antigens. Limited number of studies of V-region determinants of human anti-DNA MAbs indicate prior sensitization of lupus T cells to VH determinants and that V-region reactive T cells are not deleted in periphery of healthy individuals.

Identification of anti-thrombin antibodies in the antiphospholipid syndrome that interfere with the inactivation of thrombin by antithrombin.

The combined presence of anti-phospholipid (PL) Ab, including lupus anticoagulants (LAC) and/or anticardiolipin Ab (aCL), and thrombosis is recognized as the antiphospholipid syndrome (APS). LAC are detected as an inhibitory effect on PL-restricted in vitro blood coagulation tests, and are comprised mainly of Ab against beta(2) glycoprotein I and prothrombin (PT). Recently, anti-PT Ab (aPT) were found to be associated with thrombosis by some investigators, although this is not confirmed by others. Considering that aPT are heterogeneous in patients and that PT is converted into thrombin, we hypothesize that certain aPT in patients may bind to thrombin, and that some of such anti-thrombin Ab may interfere with thrombin-antithrombin (AT) interaction and thus reduce the AT inactivation of thrombin. To test this hypothesis, we searched for anti-thrombin Ab in APS patients and then studied those found for their effects on the AT inactivation of thrombin. The results revealed that most, but not all, aPT-positive patient plasma samples contained anti-thrombin Ab. To study the functional significance of these Ab, we identified six patient-derived mAb that bound to both PT and thrombin. Of these mAb, three could reduce the AT inactivation of thrombin, whereas others had minimal effect. These findings indicate that some aPT in patients react with thrombin, and that some of such anti-thrombin Ab could inhibit feedback regulation of thrombin. Because the latter anti-thrombin Ab are likely to promote clotting, it will be important to develop specific assays for such Ab and study their roles in thrombosis in APS patients.

Molecular characterization of a highly divergent HIV type 1 isolate obtained early in the AIDS epidemic from the Democratic Republic of Congo.

Numerous complete human immunodeficiency virus type 1 (HIV-1) genomes have been characterized for contemporary viruses, but few isolates obtained early in the HIV-1 epidemic have been studied. In this article, we describe the molecular characterization of an HIV-1 isolate (83CD003) that was obtained from an AIDS patient in Kinshasa, Democratic Republic of Congo (DRC) in 1983. The complete 83CD003 genome was sequenced in its entirety and found to encode uninterrupted open reading frames for all viral genes. Phylogenetic analysis revealed 83CD003 was a member of the major (M) group of HIV -1, but did not group with any of the known subtypes. Rather, it formed an independent lineage in all regions of its genome that was roughly equidistant from representatives of all other subtypes. Similarly, 83CD003 also did not cluster with any of several unclassified group M sequences that have been reported more recently to circulate in the DRC, suggesting that it may represent an early group M lineage thai is either rare or has gone extinct. The molecular clone of 83CD003 yielded an infectious virus after transfection into mammalian cells and its biological properties can be further studied.

Wild Mandrillus sphinx are carriers of two types of lentivirus.

Mandrillus sphinx, a large primate living in Cameroon and Gabon and belonging to the Papionini tribe, was reported to be infected by a simian immunodeficiency virus (SIV) (SIVmndGB1) as early as 1988. Here, we have identified a second, highly divergent SIVmnd (designated SIVmnd-2). Genomic organization differs between the two viral types; SIVmnd-2 has the additional vpx gene, like other SIVs naturally infecting the Papionini tribe (SIVsm and SIVrcm) and in contrast to the other SIVmnd type (here designated SIVmnd-1), which is more closely related to SIVs infecting l'hoest (Cercopithecus lhoesti lhoesti) and sun-tailed (Cercopithecus lhoesti solatus) monkeys. Importantly, our epidemiological studies indicate a high prevalence of both types of SIVmnd; all 10 sexually mature wild-living monkeys and 3 out of 17 wild-born juveniles tested were infected. The geographic distribution of SIVmnd seems to be distinct for the two types: SIVmnd-1 viruses were exclusively identified in mandrills from central and southern Gabon, whereas SIVmnd-2 viruses were identified in monkeys from northern and western Gabon, as well as in Cameroon. SIVmnd-2 full-length sequence analysis, together with analysis of partial sequences from SIVmnd-1 and SIVmnd-2 from wild-born or wild-living mandrills, shows that the gag and pol regions of SIVmnd-2 are closest to those of SIVrcm, isolated from red-capped mangabeys (Cercocebus torquatus), while the env gene is closest to that of SIVmnd-1. pol and env sequence analyses of SIV from a related Papionini species, the drill (Mandrillus leucophaeus), shows a closer relationship of SIVdrl to SIVmnd-2 than to SIVmnd-1. Epidemiological surveys of human immunodeficiency virus revealed a case in Cameroon of a human infected by a virus serologically related to SIVmnd, raising the possibility that mandrills represent a viral reservoir for humans similar to sooty mangabeys in Western Africa and chimpanzees in Central Africa.

Virologic and immunologic determinants of heterosexual transmission of human immunodeficiency virus type 1 in Africa.

More than 80% of the world's HIV-infected adults live in sub-Saharan Africa, where heterosexual transmission is the predominant mode of spread. The virologic and immunologic correlates of female-to-male (FTM) and male-to-female (MTF) transmission are not well understood. A total of 1022 heterosexual couples with discordant HIV-1 serology results (one partner HIV infected, the other HIV uninfected) were enrolled in a prospective study in Lusaka, Zambia and monitored at 3-month intervals. A nested case-control design was used to compare 109 transmitters and 208 nontransmitting controls with respect to plasma HIV-1 RNA (viral load, VL), virus isolation, and CD4(+) cell levels. Median plasma VL was significantly higher in transmitters than nontransmitters (123,507 vs. 51,310 copies/ml, p < 0.001). In stratified multivariate Cox regression analyses, the risk ratio (RR) for FTM transmission was 7.6 (95% CI: 2.3, 25.5) for VL > or = 100,000 copies/ml and 4.1 (95% CI: 1.2, 14.1) for VL between 10,000 and 100,000 copies/ml compared with the reference group of <10,000 copies/ml. Corresponding RRs for MTF transmission were 2.1 and 1.2, respectively, with 95% CI both bounding 1. Only 3 of 41 (7%) female transmitters had VL < 10,000 copies/ml compared with 32 of 93 (34%) of female nontransmitters (p < 0.001). The transmission rate within couples was 7.7/100 person-years and did not differ from FTM (61/862 person-years) and MTF (81/978 person-years) transmission. We conclude that the association between increasing plasma viral load was strong for female to male transmission, but was only weakly predictive of male to female transmission in Zambian heterosexual couples. FTM and MTF transmission rates were similar. These data suggest gender-specific differences in the biology of heterosexual transmission.