Thanks to our immune systems, most of us co-exist peacefully with common microorganisms in the environment. For immunocompromised people, though, even ordinary garden fungi pose a threat.
Markus Kalkum, Ph.D., assistant professor in the Department of Immunology, and James Ito, M.D., chief of the Division of Infectious Diseases, are working together with other City of Hope investigators on a vaccine to lessen that danger.
Their target: spores of a ubiquitous, soil-dwelling fungus known as Aspergillus fumigatus, which can cause a potentially fatal lung disease called invasive pulmonary aspergillosis.
“We all inhale several hundred spores a day — usually without negative impact on our health, due to innate immunity,” Kalkum said. “However, inhalation of spores can prove fatal to bone marrow or organ transplant patients who must take immunosuppressive drugs.”
Ito says that although use of antimicrobial drugs has greatly decreased the risk of most infections, fungal infections still pose a threat. “Fungal infections have always been around, but they’ve moved up into first place now,” he said. “In fact, aspergillosis is the most deadly of all fungal infections.”
Physicians prescribe immunosuppressive corticosteroids to patients following bone marrow transplant (BMT) to combat graft-versus-host disease, in which the patient’s immune system sees the transplanted cells as “foreign” and tries to kill them. But corticosteroids disable some of the very immune cells whose job it is to rid lungs of fungal invaders.
One solution might be to treat infected BMT patients with antifungal drugs. Unfortunately, though, these drugs are only partially effective. Following antifungal treatment, Ito said, “the best response we have ever got in BMT patients against Aspergillus was 31 percent — the rest of the patients with invasive fungal infections succumbed to the disease. We felt there had to be another approach.”
So in 2002, Ito and colleague Joseph Lyons, Ph.D., published a study in the Journal of Infectious Diseases demonstrating that an Aspergillus vaccine could work, even in immunosuppressed animals. They “vaccinated” mice with a crude mix of pulverized Aspergillus proteins, treated the mice with corticosteroids as BMT patients would be and then exposed mice to fungal spores. “A hundred percent of the mice died if we didn’t vaccinate them,” reported Ito, “but we had a significant survival rate if we did.”
Recently, Kalkum, in a collaborative study with Ito published in Infection and Immunity, used the analytical technique of mass spectrometry to detect a specific protein in that fungal vaccine soup that stimulated the immune response. When they vaccinated mice with just that protein, it, like the mix, protected mice exposed to infectious Aspergillus.
However, an obstacle remained. “The problem with this protein is that it is a known allergen,” said Kalkum. Using standard gene manipulation techniques, Kalkum’s group snipped off parts of the protein and revaccinated the mice, a strategy that proved successful. “We produced modified forms that lack the allergenic parts and showed that they were still able to protect mice,” Kalkum said.
The investigators now aim to establish antifungal immunity in immunodeficient humans, so they are looking at human immune responses to mold proteins. “Results from our animal experiments are having a great impact on experiments on human samples, since they tell us which antigens to test,” Kalkum added.
Once a vaccine is developed, questions remain as to how to administer it. “It may turn out that it’s not the patient, but rather the bone marrow donor, who will need to be immunized,” Kalkum said, so that the donor’s immune cells can combat the fungus in the recipient. Organ donor recipients could be immunized before transplantation.
Development of an Aspergillus vaccine is a highly collaborative effort at City of Hope. Sharon Wilczynski, M.D., Ph.D., professor of pathology in the Division of Anatomic Pathology, has helped in obtaining and analyzing tissue samples, and Michael Kalos, Ph.D., who directed the Clinical Immunobiology Correlative Studies Laboratory, assisted with analysis of signaling proteins upregulated in vaccinated mice.