HPH NOW, Mar 16, 2007, HSPH Team Identifies Noninfectious Pathway for HIV

J. Roberto Trujillo

HIV is a crafty virus. It attacks the body by invading and taking over the very cells meant to protect humans from infection. There - hiding within cells such as macrophages and lymphocytes - the virus uses the body's natural machinery to replicate itself, destroying the immune system and leaving patients open to a range of debilitating and deadly opportunistic infections.

Now, a team led by HSPH researchers has described a previously unappreciated pathway used by HIV to enter macrophages and has shown that the virus, once in the cells through this entryway, doesn't appear to replicate. Rather than causing infection, the virus is destroyed, and an immune response may be triggered. The researchers suggest that these findings not only provide a better understanding of how HIV interacts with human cells, but also offer new considerations for HIV vaccine design. Their paper will appear in the March 20 print issue of the Proceedings of the National Academy of Sciences.

The paper's lead author is J. Roberto Trujillo, a former HSPH investigator of neurovirology and cell biology, who now works as the head of the Laboratory of Neurovirology at the Institute of Human Virology, University of Maryland. He conducted the research for the paper while at HSPH. His co-authors are Joseph Brain, Max Essex, Rick Rogers, Ramon Molina, and Mary Fran McLane, all of HSPH, and Fernando Dangond of Brigham and Women's Hospital.

Joseph Brain

"This paper represents a highly productive interaction between HSPH scientists interested in virology and those focused on macrophage biology," said Brain, senior author and Cecil K. and Philip Drinker Professor of Environmental Physiology in the HSPH Department of Environmental Health. "We have shown that a primitive, non-specific receptor is relevant to host defenses against the HIV virus."

Macrophages are the immune system equivalent of a voracious eater, wandering the body in search of pathogens or worn-out cells. They engulf and digest intruders and unwanted cells and debris. Macrophages also regulate key aspects of immunity and inflammation.

HIV expresses a glycoprotein on its surface called gp120, whose molecular weight is half protein and half carbohydrate. AIDS researchers have long emphasized the protein portion of gp120 - which interacts with CD+4 and CCR5 co-receptors on host cells - because viral proteins open an avenue for HIV to inject its genetic material.

But no one has adequately characterized the role of the carbohydrate part of gp120 in facilitating an alternate entry pathway. The HSPH team has found that HIV-1, presenting carbohydrates on its surface, binds to mannose receptors on macrophages and on similar central nervous system cells called microglia. The mannose receptors then become the doorways into the cells, leading to viral degradation instead of viral replication.

How? When HIV-1 invades a cell via mannose receptors, the virus enters a phagocytic pathway, which means that the pathway is designed to engulf foreign matter. The host cell consumes the virus whole, forming a vesicle called an endosome around the virus. The newly formed endosome attracts smaller organelles called lysosomes in the host cell's cytoplasm that brim with digestive enzymes. The lysosomes fuse with the endosome, and the virus finds itself in an "intracellular stomach," getting broken down and degraded by the enzymes. The viral genetic material never reaches the nucleus of the host cell. Instead, pieces of the virus may be preserved and presented by macrophages to the immune system, which can then lead to an antibody response.

In contrast, when the virus enters via CD+4 and CCR5 co-receptors, HIV-1 does not enter a phagocytic pathway, nor does it enter as a whole virus. Instead, HIV-1 docks with the co-receptors and injects its viral core directly into the host cell's cytoplasm. The virus' genetic material is then free to integrate with human DNA, which sets the stage for viral replication. No endosome is created around the virus, and no lysosomes come to digest the virus.

The researchers hypothesize that a vaccine may be able to take advantage of these new insights. Hypothetically, a version of gp120 could be developed that does not target CD+4 and CCR5 co-receptors, but instead prefers mannose receptors, theoretically dooming the virus.

Max Essex

The researchers encourage further examination of the mannose receptor pathway. Such investigations may shed new light on human immune system defense mechanisms and on the dynamics of the pathogenesis of HIV/AIDS, they suggest.

"We already knew that the amount and deposition pattern of mannose on gp120 is related to virulence for the HIV group of viruses. This may help to explain why," said Max Essex, chair of the HSPH AIDS Initiative.

Said Trujillo, "This non-infectious pathway of HIV-1 by the mannose receptor leads to non-integration of HIV-1 into the chromosomes, a characteristic step of HIV-1 replication. These findings open the possibility of exploring this natural pathway in order to deliver a vaccine vector for HIV-1 without integration into the host's DNA, an important criterion for sterile immunity. In other words, we want to vaccinate people without infecting them with the virus. More studies need to be conducted in order to understand if this non-infectious pathway can generate effective viral immunity."