Posted by Jef Akst

[Entry posted at 20th July 2009 06:00 PM GMT]

 

New findings have disproved a leading explanation for why an experimental HIV vaccine made subjects more susceptible to the virus, reopening the door for further HIV vaccine efforts based on similar principles.

Human Immunodeficiency Virus Image: NIAID

The Merck-funded STEP study, which used an adenoviral vector to deliver an HIV vaccine candidate, was halted in 2007 after the data suggested the vaccine increased the risk of HIV infection. Researchers thought the effect might be due to an immune reaction to the viral vector, but two studies published online in Nature Medicine today show this is not the case.

"Both of these papers show that's not a possible explanation," said molecular immunologist David Weiner of the University of Pennsylvania, who was not involved in the research.

"Overall this is a positive and optimistic message for the field," said immunologist Dan Barouch, chief of Harvard University's Division of Vaccine Research and the lead author of one of the studies. "Other vaccine vectors can and should be further pursued in clinical trials."

Individuals who have previously been exposed to adenovirus serotype 5 (Ad5) -- a common cold virus -- have more Ad5-specific antibodies in their blood; that is, they are Ad5 seropositive. Analysis of the trial suggested that Ad5-seropositive subjects contracted HIV at a higher rate when they received the vaccine as opposed to the placebo. Researchers hypothesized that Ad5-seropositive individuals might be more sensitive to the Ad5 vector, and consequently, could "mount a more rapid, larger immune response," explained molecular biologist Alan Bernstein, executive director of Global HIV Vaccine Enterprise, who was not involved in the research. Because HIV attacks activated T cells, increased T-cell counts would provide more plentiful targets for HIV infection, and could thereby explain the boost in susceptibility.

The current studies analyzed frozen blood samples from two different precursor trials to the STEP study, and in both cases, the research teams disproved the idea that prior exposure to the Ad5 vector equated to higher T-cell counts. In the study led by Barouch, approximately 90% of subjects had Ad5-specific T cells prior to vaccination, but very few had Ad5-specific antibodies. This demonstrated that the two measures of immunity -- Ad5-specific antibodies and Ad5-specific T cells -- are not correlated, meaning that that Ad5 seropositivity cannot be used to predict levels of Ad5-specific T cells prior to vaccination.

Furthermore, while all subjects showed an expansion of Ad5-specific T cells after vaccination -- a typical immune response to the viral vector -- this effect was not greater in Ad5-seropositive individuals. Just the opposite, Barouch's team found 2-3 fold more Ad5-specific T cells in Ad5-seronegative subjects, suggesting that Ad5 seropositivity did not raise the T cell count.

Finally, using intracellular cytokine staining to identify individual cells, immunologist Michael Betts of the University of Pennsylvania School of Medicine and his colleagues found in the second study that the T cells produced post-vaccination did not differ in the types of cytokines they secreted. This suggested there were no functional differences in the T cells produced by seropositive and seronegative individuals.

"The two papers really biologically have shown that there's no relationship between adenovirus 5 seropositivity and increased acquisition [of HIV]," said Bernstein. "[This] result really rules out the possibility that it was the vaccine itself, and the fact that we used Adeno5, that was somehow increasing susceptibility to acquiring [HIV] in those volunteers."

As for what caused the increase in HIV susceptibility in the STEP study, "that's the $64 million question," Weiner said. One important next step is to look at these same immune response measures in the body's mucosal tissues instead of blood. For one thing, "90% of all memory T cells are found in intestinal mucosa," said Betts. "In primary HIV infection, the vast majority of all viral replication takes place in the gut because that's where the majority of T cells are." Furthermore, "that's where the infection occurs," added molecular biologist Nelson Michael of the Walter Reed Army Institute of Research. "The fact that you don't see [a difference in T cell] activation in the blood may not necessarily mean that you wouldn't have seen [differences in] activated T cells [in the mucosal tissues of the genitalia]."

Alternatively, there may not be a correlation between Ad5 seropositivity and HIV infection after all. In addition to the Ad5 seropositive individuals, uncircumcised men who received the STEP study vaccine also contracted HIV at a higher rate than those receiving the placebo. "Circumcision and Ad5 seropositivity were both factors," Michael explained, "but [they] confounded one another." With more data rolling in from the now-unblinded STEP study subjects, he said, it appears that "the effect of Ad5 [seropositivity] washes out when you control for circumcision."

Whatever the reason for the unexpected results, the STEP study and its aftermath were "an important lesson for this field," said Bernstein. "The mistake was thinking the trial failed because the candidate failed. We shouldn't mix up the two. The trial will be a success if we learn something from it, [and] I think that's turning out to be the case."