AP-3 in dendritic cells. Credit: Michael Marks, PhD, Perelman School of Medicine; Immunity
A year ago this month, Penn Medicine announced the formation of the Penn Center for Orphan Disease Research and Therapy. Perelman School of Medicine researchers are currently conducting nearly 300 research projects on rare/orphan diseases, with nearly a third of those at the preclinical stage, looking at molecular intricacies at the cellular level before a full-blown clinical trial.
Case in point: A research article in a recent issue of Immunity from the lab of Mickey Marks, PhD, professor of Pathology and Laboratory Medicine, in part explains the recurrent bacterial infections in patients with a rare genetic disease called Hermansky-Pudlak Syndrome (HPS) type 2. Symptoms include albinism, prolonged bleeding, pulmonary fibrosis, and recurrent infections by both viruses and bacteria. Previous work on these patients and related mouse models had documented defects in killer T cells that attack virally infected cells and in other cell types that produce antiviral factors, but the recurrent bacterial infections had remained unexplained.
The immune dysfunction is related to helper T cells, which in this case activate inflammatory cells like macrophages and neutrophils, making them more effective microbe killers. Mutations in a protein in another immune cell that presents pieces of the invader’s protein, or antigen, to host immune cells underlies the inability to activate the helper T cells in Hermansky-Pudlak Syndrome type 2. If this sentinel system is not working correctly, frequent and recurring infections occur.
Hermansky-Pudlak Syndrome-related genes encode proteins that are parts of protein complexes involved in moving molecules between different internal vesicles, or membraned compartments, within a cell. HPS type 2 patients, in particular, lack a protein complex called adaptor protein-3. AP3 is also defective in a mouse model called the pearl mouse due to the white color of its coat.
The Penn researchers show that AP3 is required by dendritic cells so they can properly present bacterial antigens on their surface to helper T cells, which coordinate inflammatory responses to bacterial infections.
To test whether this requirement has an impact on antibacterial immunity, pearl mice were infected with a bacterium called Listeria monocytogenes -- the causative agent of listeriosis -- that was engineered to express the egg white protein ovalbumin. (This expression is a trick researchers use so that T cells recognize a specific antigen and aren’t themselves affected by the AP3 mutation.)
The team found that T helper cells in infected pearl mice were stimulated more slowly than in normal mice, and those that were stimulated were less effective in coordinating the antibacterial inflammatory response.
In addition, using a different system, they found that when AP3-deficient dendritic cells ingested bacteria-like particles that were coated with antigens, the antigens were processed but not moved to the cell surface for presentation to T cells. So, the T cells were never educated to be able to see the mark of the bacteria.
To explain this problem of antigen presentation, the team looked at toll-like receptors (TLRs). TLRs are proteins that span the cell membrane and recognize molecules that are commonly found on microbes. When microbes make it past the skin or the intestinal tract, the next line of defense are the TLRs, which activate inflammatory responses. Among other things, TLRs on dendritic cells, and other cell types, stimulate antigen presentation to initiate immune responses.
Although the team found that TLRs were normally expressed on the surface of pearl mouse dendritic cells, there were fewer TLRs on the intracellular compartments in which ingested bacteria accumulate. What’s more, bacteria-like particles were less able to stimulate TLRs in pearl dendritic cells compared to the dendritic cells of normal mice. The team surmises that AP3 is needed to deliver TLRs to these compartments, and that signals from these compartments stimulate antigen presentation.
If anything goes wrong in this complicated process, disorders like Hermansky-Pudlak Syndrome can arise.
While the study might explain why HPS2 patients suffer from recurrent bacterial infections, it also raises a number of new questions that the team is now trying to address. How does AP3 carry TLRs from other sites within dendritic cells to the bacteria-laden compartments? How do TLR signals from these compartments differ from the signals from the cell surface, which work just fine without AP3? How do these signals stimulate antigen presentation? How in general do bacterial antigens get from the intracellular compartments to the cell surface? Finally, does this process occur only in dendritic cells, or does it occur in such other immune cell types as macrophages? Answers to these questions might be important in controlling other bacterial infections, such as Salmonella, the causative agent of food-borne diarrhea, or Legionella, the causative agent of Legionaire’s disease.
Mantegazza AR, Guttentag SH, El-Benna J, Sasai M, Iwasaki A, Shen H, Laufer TM, & Marks MS (2012). Adaptor protein-3 in dendritic cells facilitates phagosomal toll-like receptor signaling and antigen presentation to CD4(+) T cells. Immunity, 36 (5), 782-94 PMID: 22560444