Caught on scans: Immune cells nab parasites

Amanda D. Francoeur, amanda.francoeur@laurin.com

While observing immune cells detain parasitic intruders that were injected into the skin of animal models – routine for any immune system – scientists at the University of Pennsylvania discovered that fluorescence imaging allowed them to see a more detailed view of cells patrolling through the tissue, recognizing and apprehending an infection for the first time.

“We need to understand which cells are involved in generating an immune response against these parasites,” said lead investigator Dr. Wolfgang Weninger, now a professor at Centenary Institute of Cancer Medicine and Cell Biology at the University of Sydney in Australia. “A dynamic view of the interactions between these cells and parasites allows us to better understand the orchestration of immune responses.”

A specific subset of immune cells – called dermal dendritic cells, or DDCs – in the second, or dermis, layer of skin function by recognizing and revealing foreign antigen on their surface to inform other immune cells of the infection.

Dermal dendritic cells (yellow fluorescent protein) in the dermis of skin slow their progression through the interstitial tissue space and grow pseudopods to detain foreign parasites (red fluorescent protein). Researchers now can observe, in detail, immune cells capturing an infection in the skin.

In a paper published online November 2008 in the Public Library of Science Pathogens, the researchers report using genetically modified mice to produce yellow fluorescent protein specifically in DDCs. The fluorescence helped to identify DDCs and to track their behavior in space and time. Two-photon microscopy, which uses a near-infrared laser to excite fluorescent probes with little phototoxicity, was used to view living tissue. The study revealed that the DDCs, which were most abundant at 5 to 20 μm, just below the outermost epidermal layer, moved continuously through the dermis in the absence of infection.

The scientists then injected Leishmania major into the dermis layer of the ears of the mice. Distributed by bites from sand flies, the parasite is common in the Middle East. Troops in Iraq and Afghanistan, where the flies are prevalent, have been susceptible to the disease, and skin sores are typical symptoms.

Twenty minutes after injecting L. major into the mice, there was considerable change in the DDCs’ movement. Instead of continuing through the dermis, their mobility stalled, and they seized the infected cells. They then grew highly motile cell protrusions, or dendrites, that wrapped around the infected cells.

The researchers found that the dendrites moved toward the infection at about 2.5 μm per minute and reached up to 50 μm in length. Two to three hours after injection, about 70 percent of the DDCs had absorbed one or more of the parasites.

To test whether the DDCs specifically recognized the infectious contaminants in the skin, the researchers also injected fluorescent latex beads and found that only about 20 percent interacted with the beads. When the beads were injected simultaneously with the parasites, the DDCs went after the parasites only.

The discovery of how the immune system reacts could help researchers develop vaccines to protect against these types of infections.

“We are currently looking as to how parasites are picked up by DDC,” Weninger said. “Once we identify a specific molecule, we will proceed to design vaccines.”

Studying the interception of parasites also could provide researchers with an application to manage Trypanosomiasis, also a member of the L. major microbe family but capable of much more damage when contracted. The parasite can trigger African sleeping sickness, which, if untreated, can be fatal.