Cell-associated HIV mucosal transmission: The neglected pathway

Dr. Deborah Anderson from Boston University School of Medicine (BUSM) and her colleagues are challenging dogma about the transmission of the human immunodeficiency virus type 1 (HIV-1). Most research has focused on infection by free viral particles, while this group proposes that HIV is also transmitted by infected cells. While inside cells, HIV is protected from antibodies and other antiviral factors, and cell-to-cell virus transmission occurs very efficiently through intercellular synapses. The Journal of Infectious Diseases (JID) has devoted their December supplement to this important and understudied topic.

The 10 articles, four from researchers at BUSM, present the case for cell-associated HIV transmission as an important element contributing to the HIV epidemic. Anderson chides fellow researchers for not using cell-associated HIV in their transmission models: "The failure of several recent vaccine and microbicide clinical trials to prevent HIV transmission may be due in part to this oversight."

Approximately 75 million people in the world have been infected with HIV-1 since the epidemic started over 30 years ago, mostly through sexual contact and maternal-to-child transmission. A series of vaccine and microbicide clinical trials to prevent HIV transmission have been unsuccessful, and scientists are returning to the drawing board to devise new approaches. The JID supplement advocates for new strategies that target HIV-infected cells in mucosal secretions.

The publication presents evidence that HIV-infected cells populate genital secretions from HIV-infected men and women as well as breast milk, and genetic evidence suggesting that cell-associated HIV transmission occurs in people. Various models for studying cell-associated HIV transmission and molecular targets for intervention are also presented. Finally, the efficacy of current HIV prevention strategies against cell-associated HIV transmission and opportunities for further development are described.

Bone metastases in prostate cancer blocked by HIV drug

The receptor CCR5, targeted by HIV drugs, is also key in driving prostate cancer metastases, suggesting that blocking this molecule could slow prostate cancer spread

Although prostate cancer can be successfully treated in many men, when the disease metastasizes to the bone, it is eventually lethal. In a study researchers show that the receptor CCR5 best known for its role in HIV therapy, may also be involved in driving the spread of prostate cancer to the bone.

"Because this work shows we can dramatically reduce metastasis in pre-clinical models, and because the drug is already FDA approved for HIV treatment- we may be able to test soon whether this drug can block metastasis in patients with prostate cancer," says Richard Pestell, M.D., Ph.D., MBA, Director of the Sidney Kimmel Cancer Center at Thomas Jefferson University and senior author on the study.

The work builds on previous research from Dr. Pestell's lab that showed in 2012 that CCR5 signaling was key in the spread of aggressive forms of breast cancer to the lungs. Their prior paper demonstrated that breast cancer cells that carried the CCR5 receptor on their surface were drawn to the lung. Given that prostate cancer cells were attracted to the bone and brain, Pestell's team investigated whether CCR5 could play a role in prostate cancer metastases as well.

The research was complicated by the fact that there was no immune competent mouse model of prostate cancer that reliably developed bone and brain metastases. So the researchers developed a prostate cancer cell line, driven by an upregulated Src gene, that regularly caused bone metastases in immune-competent mouse models. Because the immune system is so important in human prostate cancer it was important to develop a model that reflected human disease.

The researchers analyzed the genes of the metastasized bone and brain tumors and found genes driving the cancer were also involved in the CCR5 signaling pathway. To investigate further, the researchers administered the CCR5-blocking drug maraviroc to the new prostate cancer mouse model. In comparison to control animals, maraviroc dramatically reduced the overall metastatic load by 60 percent in the bone, brain and other organs.

Finally, in order to determine whether a similar mechanism might be at play in human prostate cancer, the researchers mined the genomic data of patients with prostate cancer and found that CCR5 was more highly expressed in prostate cancer tissue compared with normal tissue, and even more highly expressed in metastases compared with primary tumors. "In fact, we noticed that patients who had a lower expression of the CCR5-pathway genes had a longer survival times, whereas high expression of these CCR5 genes was associated with a shorter overall survival," said co-first author Xuanmao Jiao, Ph.D., and an instructor in the department of Cancer Biology at Jefferson.

Hepatitis C ruled out as cause of mental impairment in HIV patients

To stop these changes, scientists have to learn what is causing them. One possibility researchers are considering is that long-term infections with other pathogens, common in HIV-positive patients, are affecting the brain. But a new study has eliminated one of their prime suspects: the hepatitis C virus, which infects about one in every three HIV-positive patients in the United States.

The research, conducted by a team that includes scientists at Washington University School of Medicine in St. Louis, appeared Dec. 10 in Neurology.
"Hepatitis C infection has serious long-term side effects, such as damage to the liver, but our research indicates that it does not affect the brain," said lead author David Clifford, MD, of Washington University.

The research was conducted as part of the CNS HIV Anti-retroviral Therapy Effects (CHARTER) study, a multicenter collaborative that is examining the long-term neurological effects of HIV infection.

Hepatitis C most commonly infects illicit-drug users who share needles used to inject the drugs. Drug abuse can harm the brain, making it difficult to determine whether hepatitis C or problems caused by drug use contribute to brain impairment in patients with both HIV and hepatitis C.

To answer this question, Clifford and his colleagues studied 1,582 HIV patients, 408 of whom were also infected with hepatitis C. Each patient received a detailed neuropsychological exam devised by Clifford and other CHARTER researchers to detect signs of HIV-associated mental deficits.

The exam takes two to 2 1/2 hours, and includes written examinations taken by the patient and physical exams given by medical professionals. Patients are tested for their ability to express themselves, to make decisions, to learn and retain new information using multiple types of memory, and to move the body and control muscles.

"In all, we looked at seven domains of mental function," said Clifford, who is the Melba and Forest Seay Professor of Clinical Neuropharmacology in Neurology. "We studied their overall performance and looked at each domain individually and found no evidence that the group with hepatitis C performed worse."

According to Clifford, this was particularly impressive because the participants in the group with hepatitis C were older, had less education and had lower scores on tests of reading, comprehension, spelling and math.

With hepatitis C eliminated, Clifford and his colleagues are turning their attention to the immune responses triggered by HIV in the brain and the bowel during the initial stages of infection. He and others believe these early responses, which include bursts of inflammation, lead to chronic inflammation that adversely affects the brain.

"If a hepatitis C infection gets to the point where it damages liver function, the resulting inflammation might well contribute to mental impairment," Clifford said. "Beyond that, though, it doesn't seem to be an active collaborator in the harm HIV does to the brain."

People with mental illness more likely to be tested for HIV

People with mental illness are more likely to have been tested for HIV than those without mental illness, according to a new study. The researchers also found that the most seriously ill – those with schizophrenia and bipolar disease – had the highest rate of HIV testing

The study assessed nationally representative data from 21,785 adult respondents from the 2007 National Health Interview Survey (NHIS) and provides an update of prior research using 1999 and 2002 NHIS data. The 2007 version is the most recent cycle of the survey that included information both on mental health diagnoses and HIV testing.

The current Penn-led study adds precision to earlier research by reporting on HIV-testing rates according to specific mental health diagnoses; previous studies did not differentiate persons with, for example, depression, bipolar disorder, and schizophrenia spectrum disorder.

The researchers found that 15 percent of respondents reported a psychiatric disorder. Of these, 89 percent had symptoms of depression and/or anxiety, 8.5 percent had bipolar disorder, and 2.6 percent had schizophrenia spectrum disorder. Among persons reporting at least one mental illness, 48.5 percent had been tested for HIV. The 48.5 percent rate compares to a testing rate of 35 percent among those without mental illness. More specifically, 64 percent of persons with schizophrenia, 63 percent of persons with bipolar disorder, and 47 percent of persons with depression and/or anxiety reported ever being tested for HIV.

"Our study shows that persons with mental illness and/or their care providers recognize that they are at higher risk and should be tested," said senior author Michael B. Blank, PhD, associate professor in Psychiatry at Penn and co-director of the Penn Mental Health AIDS Research Center. "However, by no means we should be complacent since these results may in large part be due to individual vigilance. The fact is there are few formal prevention and screening efforts targeted at this at-risk population. In light of the fact that mentally ill people are more likely to engage in risky behavior, mental health providers should consider routinely offering HIV/AIDS testing, something that does not typically occur now."

HIV infection and mental illness are often co-occurring health conditions, with nearly half of persons living with HIV having a psychiatric disorder while between 5-23 percent of those with mental illness are infected with HIV.

In addition, the study found that persons aged 25-44, women, racial and ethnic minorities, individuals who are windowed/divorced/separated, those reporting excessive use of alcohol or tobacco, and persons with HIV risk factors were significantly more likely to be tested for HIV than their counterparts.
Separate research has found that mentally ill individuals are more likely than others to engage in high-risk behaviors associated with HIV transmission, including unprotected sexual intercourse, injection drug use, and sex with multiple partners.

"Our finding that persons with mental illness were tested for HIV at a higher rate than those without mental illness is encouraging and consistent with previous analyses," said lead author Baligh R. Yehia, MD, MPP, MSHP, assistant professor of Medicine at the Perelman School of Medicine at the University of Pennsylvania and director of the Penn Medicine Program for LGBT Health. "However, the large number of people with mental illness who still have not been tested necessitates increased public health prevention efforts, particularly in light of the increased HIV risk in this population."

The CDC recommends that all persons aged 13-64 be tested for HIV in healthcare settings and that persons with increased risk such as injecting drug users and their sex partners, sex workers, men who have sex with men, and heterosexuals with multiple sex partners be tested at least annually.

HIV/AIDS drugs could be repurposed to treat AMD, researchers suggest

Drugs that have been used for the past 30 years to treat HIV/AIDS, could be repurposed to treat the dry form of age-related macular degeneration (AMD), a new study suggests. Age-related macular degeneration is a progressive condition that is untreatable in up to 90 percent of patients and is a leading cause of blindness in the elderly worldwide. The two forms of age-related macular degeneration, wet and dry, are classified based on the presence or absence of blood vessels that have invaded the retina.

Age-related macular degeneration is a progressive condition that is untreatable in up to 90 percent of patients and is a leading cause of blindness in the elderly worldwide. The two forms of age-related macular degeneration, wet and dry, are classified based on the presence or absence of blood vessels that have invaded the retina. A detailed understanding of the molecular mechanisms underlying wet age-related macular degeneration has led to several robust FDA-approved therapies. In contrast, there are no approved treatments for dry age-related macular degeneration thus far.

Nucleoside reverse transcriptase inhibitors (NRTIs) are the most widely used class of anti-HIV drugs. NRTIs are thought to be therapeutic in HIV/AIDS patients because they target the enzyme reverse transcriptase, which is critical for replication of HIV. Previous work from the Ambati lab found that a type of toxic molecule called Alu RNA accumulate in the retina to cause dry age-related macular degeneration; interestingly, Alu RNA and HIV are similar in that they both require reverse transcriptase to fulfill their life cycle.

In their Science publication, Fowler et al. report that multiple FDA-approved NRTIs prevented retinal degeneration in a mouse model of dry age-related macular degeneration. Surprisingly, this effect of NRTIs in the eye was not due to the well-known function of these drugs to inhibit reverse transcriptase. Instead, NRTIs blocked an innate immune pathway called the "inflammasome," even in experimental systems in which the NRTIs were not capable of blocking reverse transcriptase. In their report, they also showed that NRTIs were effective in other disease models that share common signaling pathways with the dry age-related macular degeneration model, including the "wet" form of age-related macular degeneration -- a disease that when treated still does not lead to substantial vision improvement in two-thirds of patients -- and graft-versus-host disease which is the major obstacle preventing successful allogeneic hematopoietic stem cell transplantation.

"Repurposing of NRTIs could be advantageous, for one, because they are very inexpensive. Moreover, through decades of clinical experience, we know that some of the drugs we tested are incredibly safe. Since these NRTIs are already FDA-approved, they could be rapidly and inexpensively translated into therapies for a variety of untreatable or poorly treatable conditions," said Benjamin Fowler, the lead author and a postdoctoral fellow in the Ambati lab. Ambati added, "We are excited at the prospect of testing whether NRTIs could be effective in halting the progression of age-related macular degeneration in patients."

NRTIs were originally designed to treat cancer in the 1960s. They re-emerged in the late 1980s and became the first drugs the FDA-approved to treat HIV/AIDS.

Semen directly impairs effectiveness of microbicides that target HIV

Researchers have discovered why microbicides developed to prevent HIV succeed in the lab but fail in clinical trials: Semen. Semen enhances the infectiousness of HIV by causing the virus to cluster together, increasing its ability to attach to and infect cells. This effect is then sufficient to override the antiviral properties of the microbicides.

In the fight against HIV, microbicides -- chemical compounds that can be applied topically to the female genital tract to protect against sexually transmitted infections -- have been touted as an effective alternative to condoms. However, while these compounds are successful at preventing transmission of the virus in a petri dish, clinical trials using microbicides have largely failed. A new study from the Gladstone Institutes and the University of Ulm now reveals that this discrepancy may be due to the primary mode of transportation of the virus during sexual transmission, semen.

"We think this may be one of the factors explaining why so many drugs that efficiently blocked HIV infection in laboratory experiments did not work in a real world setting," explains co-first author Nadia Roan, PhD, a visiting scientist at Gladstone and an assistant professor-in-residence in the Department of Urology at the University of California, San Francisco. "We've shown previously that semen enhances HIV infection, but this is the first time we've shown that this activity markedly reduces the antiviral efficacy of microbicides."

Semen markedly enhances the infectiousness of HIV through the presence of protein aggregates called amyloid fibrils. HIV binds to these fibrils, causing the virus to cluster together and increasing its ability to attach to and infect cells in the host -- in this case the sexual partner of the infected individual. This effect is then sufficient to increase the infectiousness of the HIV virus, thereby diminishing the antiviral properties of the microbicides.

In the study researchers tested the effectiveness of several different types of microbicides targeting the HIV virus on cells that had been exposed to HIV alone compared with cells that were treated with both HIV and semen. Across the board, they saw that not only did the cells with semen have rates of HIV infection approximately ten-fold higher than normal, these microbicides were up to twenty times less effective at blocking the virus in these cells than in those not exposed to semen.

Senior author Jan Munch, PhD, from the University of Ulm says, "Our findings suggest that targeting amyloids in semen is an alternative strategy to improve drug efficacy. The next step is to create a compound or cocktail of drugs that targets both the HIV virus and these amyloid fragments and to test its effectiveness. Also, given that semen is the main means of transmission of HIV, future testing of microbicides in the lab should be performed in the presence of semen to better predict antiretroviral efficacy in real life."

To test that it was the HIV-enhancing ability of semen that was having this effect on the microbicides and not some other substance, the researchers repeated the experiments using semen from men whose semen does not enhance HIV infection due to a disorder called ejaculatory duct obstruction. In the presence of these samples, there was no decrease in effectiveness of the anti-viral microbicides, confirming the importance of the HIV-promoting effects of semen in counteracting the effectiveness of these drugs.

Most microbicides work by targeting the virus itself, attempting to break it down or blocking its ability to infect a cell. However, the heightened infectiousness of HIV in the presence of semen appears to over-power any anti-viral effects the microbicides possess. The one exception to this finding is a different type of microbicide that acts on the host cells' receptors, stopping the virus from latching on from within. In the current study, this microbicide, called Maraviroc, was equally effective in preventing infection both with and without the presence of semen.

"There are important potential clinical implications for this study," says Warner Greene, MD, PhD, director of the Gladstone Institute of Virology and Immunology and a senior author on the paper. "Microbicides were originally developed as a way to empower and protect women in sub-Saharan Africa who often don't have a way to negotiate safe sex or condom use. However, the first generation of microbicides were largely ineffective or worse, some even leading to increased transmission of the virus. This study sheds light on why these microbicides did not work, and it provides us with a way to fix this problem by creating a new compound drug combining antivirals and amyloid inhibitors."

Altered milk protein can deliver aids drug to infants

A novel method of altering a protein in milk to bind with an antiretroviral drug promises to greatly improve treatment for infants and young children suffering from HIV/AIDS, according to a researcher.

That's critical because an estimated 3.4 million children are living with HIV/AIDS, the World Health Organization reports, and nine out of 10 of them live in resource-limited countries in sub-Saharan Africa, where effective antiretroviral treatments still are not widely accessible or available. International medical experts believe less than a third of affected children worldwide receive an antiretroviral drug.

Complicating treatment is that most antiretroviral drugs are not well tolerated by very young children. One of the most commonly prescribed antiretroviral drugs for treating and preventing HIV infection, Ritonavir, has undesirable side effects and important oral-delivery problems. Its physicochemical properties challenge its administration to infants, explained Federico Harte, associate professor of food science.

"Ritonavir has a high hydrophobicity and low solubility in water, which lead to a low dissolution rate in the gastrointestinal fluid and, hence, to insufficient bioavailability. The liquid formulation used to treat infants over one month of age contains 43 percent ethanol and has an awful flavor that has been described as bitter-metallic, medicinal, astringent, sour and burning," he said.

"Moreover, when coming into contact with the stomach mucosa, Ritonavir causes nausea, vomiting and diarrhea. Therefore, we need to develop alternative pediatric formulations of Ritonavir and overcome its poor water solubility to improve its oral administration to infants and children."

To solve that problem, Harte looked to a group of proteins in cow's milk celled caseins. Casein proteins form spherical aggregates called casein micelles, which are responsible, incidentally, for the white color of milk. The casein micelles in mammals' milk are natural delivery systems for amino acids and calcium from mother to young, and Harte reasoned, might deliver Ritonavir molecules as well.

"I have been working with bovine casein micelles for a few years now, and we have investigated the structure and functionality of these proteins," he said. "What we found is these micelles are able to carry molecules that have very little solubility in water, that have low molecular weight and that are very hydrophobic -- such as Ritonavir."

Significantly, Harte discovered in his research that subjecting milk to ultrahigh-pressure homogenization enhances the binding properties of the casein micelles. In previous studies, he learned that casein micelles could be bound to triclosan -- an antimicrobial used in deodorants -- and vitamin D, which is added regularly to skim milk.

Normal milk is homogenized at 10 to 15 megapascals, he pointed out. Milk in this research was homogenized at between 400 and 500 megapascals, disassociating the casein micelles and improving the protein's binding qualities to attach to drug molecules.

"As a result of this enhanced binding of molecules, we believe a milk powder containing Ritonavir can be used as baby formula, providing a transport system for a drug that is not very soluble in water. Right now we are running tests, and we are in the final stages of an experiment in which we gave three different formulations to piglets," Harte said.

"We are taking blood serum samples every three hours to study the kinetics of the drug in the piglets," he said. "The hope is that -- and we don't have the data yet -- we find that the Ritonavir is being adequately delivered by the protein in milk. So if that works, I think we are pretty close to having a formulation that can be used with hydrophobic drugs."

Harte noted that with his proposal for research funding he included a letter from a pediatrician at St. Jude Children's Research Hospital describing the challenge of orally administering Ritonavir to infants and young children.

"She has been treating patients with AIDS, and the awful flavor and ethanol content of Ritonavir were big issues for her," he said. "I am hopeful that this may lead to an application that works against AIDS. We have not done any clinical trials yet, so we will need to get data from those trials to say for sure."

First Immature form of HIV seen at high resolution surprises researchers

The first structure of the immature form of HIV at a high enough resolution has been obtained by researchers, allowing them to pinpoint exactly where each building block sits in the virus. The study reveals that the building blocks of the immature form of HIV are arranged in a surprising way.

Sientists at the European Molecular Biology Laboratory (EMBL) in Heidelberg, Germany and collaborators from Heidelberg University, in the joint Molecular Medicine Partnership Unit, have obtained the first structure of the immature form of HIV at a high enough resolution to pinpoint exactly where each building block sits in the virus. The study reveals that the building blocks of the immature form of HIV are arranged in a surprising way.

"The structure is definitely different from what we'd expected," says John Briggs from EMBL, who led the work. "We assumed that retroviruses like HIV and Mason-Pfizer Monkey Virus would have similar structures, because they use such similar building blocks, but it turns out that their immature forms are surprisingly different from each other. At this point, we don't really know why."

Briggs and colleagues used cryo-electron microscopy to study the protein lattice that surrounds the virus' genetic material. After infecting one of the cells in our immune system, HIV replicates, producing more copies of itself, each of which has to be assembled from a medley of viral and cellular components into an immature virus. This is the form that leaves the cell. The protein building blocks that make up the virus are then rearranged into the virus' mature form, which can infect other cells.

The first cryo-electron microscopy images of immature HIV, obtained at EMBL in the 1990s, surprised researchers by showing that the virus did not have a regular symmetrical structure, as had been assumed. That meant it was going to be difficult to get a detailed picture of the structure of its protein lattice. Two decades on, by optimising both how data is collected at the microscope and how it is analysed, Florian Schur, a PhD student in Briggs' lab, has now achieved an unprecedentedly detailed structure.

With this structure in hand, scientists have a basis to probe further. They can use it to decide where to focus efforts for achieving the even greater detail needed to explore potential drug targets, for instance. It will also enable researchers to understand how mutations might influence how the virus assembles. And the techniques themselves can be applied to a variety of questions.

"This approach offers so many possibilities," says Schur. "You can look at other viruses, of course, but also at complexes and proteins inside cells, with a whole new level of detail."

In future, the EMBL scientists will use the approach to look at other viruses and at the vesicles that transport material inside cells. They also aim to push the techniques even further, to allow them to see other parts of the viral proteins that are currently beyond their reach, but which they suspect play an important role in HIV maturation.

"In the long term, we'd also like to investigate how drugs which are known to inhibit virus assembly and maturation actually work," Briggs concludes.

Can social media help stop the spread of HIV?

In addition to providing other potential benefits to public health, all of those tweets and Facebook posts could help curb the spread of HIV. Although public health researchers have focused early applications of social media on reliably monitoring the spread of diseases such as the flu, a new article tells of a future in which social media might predict and even change biomedical outcomes.

Although public health researchers have focused early applications of social media on reliably monitoring the spread of diseases such as the flu, Sean Young of the Center for Digital Behavior at the University of California, Los Angeles.

"We know that mining social media will have huge potential benefits for many areas of medicine in the future, but we're still in the early stages of testing how powerful these technologies will be," Young said.

With the right tools in place, he says, social media offers a rich source of psychological and health-related data generated in an environment in which people are often willing to share freely.

His recent work on Behavioral Insights on Big Data (BIBD) for HIV offers the tantalizing possibility that insights gleaned from social media could be used to help governments, public health departments, hospitals, and caretakers monitor people's health behaviors "to know where, when, and how we might be able to prevent HIV transmission."

Young details a social-media-based intervention in which African American and Latino men who have sex with men shared a tremendous amount of personal information through social media, including when or whether they had 'come out,' as well as experiences of homelessness and stigmatization. What's more, they found that people who discussed HIV prevention topics on social media were more than twice as likely to later request an HIV test.
In the context of HIV prevention, tweets have also been shown to identify people who are currently or soon to engage in sexual- or drug-related risk behaviors. Those tweets can be mapped to particular locations and related to actual HIV trends.

What's needed now is the updated infrastructure and sophisticated toolkits to handle all of those data, Young said, noting that there are about 500 million communications sent every day on Twitter alone. He and a team of University of California computer scientists are working to meet that challenge now.
Although privacy concerns about such uses of social media shouldn't be ignored, Young says there is evidence that people have already begun to accept such uses of social media, even by corporations looking to boost profits.

"Since people are already getting used to the fact that corporations are doing this, we should at least support public health researchers in using these same methods to try and improve our health and well being," he said. "We're already seeing increased support from patients and public health departments."

Real-life social networking prompts people to get tested for HIV

Old-school face-to-face social networking is a more effective way to identify people with HIV than the traditional referral method, suggests research. The study shows that social networking strategies - enlisting people in high-risk groups to recruit their peers to get tested – is more efficient and targeted than traditional testing and referral programs, resulting in 2-and-a-half times more positive test results.

As many as 20 percent of HIV-positive people are unaware of being infected with the virus, and therefore do not receive vital treatment. In addition to missing out on medications that can improve their prognosis and quality of life, these patients also are more likely to spread the disease to others. Therefore, encouraging people at risk for HIV to get tested is critical.

SNS programs likely are more effective because they are more proactive than traditional counseling, testing and referral (CTR) programs, which are available to anyone who wishes to be tested but do not offer the same motivation as encouragement by peers.

"A limitation of the traditional approach is that many people who are at high risk of HIV never take the initiative to get tested on their own," said Ryan Westergaard, MD, PhD, MPH, lead author of the study and assistant professor of medicine at the University of Wisconsin School of Medicine and Public Health, Madison. "Our study found that using social network strategies, in which we enlist people at high risk to encourage peers in their social networks to get tested, results in a higher proportion of positive HIV tests - making our efforts more effective and allowing us to reach the people who need it most."

In the study, researchers collected data from 45 HIV testing sites in Wisconsin over four years. Through SNS, 54 of 2,169 (2.49 percent) people tested were HIV-positive vs. 440 of 48,318 (.91 percent) of those tested through CTR.

In SNS - which is growing in large cities - people at high risk for HIV are paid an incentive (typically $10 to $20) for every person they refer who gets tested. Some experts are concerned that these programs are costly, but the new research suggests that SNS is worthwhile, because it results in a higher percentage of positive tests.

"Some SNS programs limit the numbers of people a recruiter can refer for testing, based on the assumption that that they're just signing up everyone they know to make more money, even if they're unlikely to be HIV-positive," said Dr. Westergaard. "Our study showed that, on average, the 30th or 40th person referred for testing through SNS had just as high if not a higher probability of having a positive HIV test than the first five or 10 people referred.This suggests SNS can be a cost-effective tool to increase testing in specific high risk pools, such as men who have sex with men and transgender people."