Dendritic cells, wards of the immune system, have the ability to distinguish between different types of pathogens - viruses, bacteria, fungi, whatever - and switch on specifically targeted immune-response genes against them.

"Dendritic cells," explains research immunologist Mathias Oelke, "are artificial antigen-presenting cells [aAPCs], which stimulate T lymphocytes into attacking infection. They are immune system sentries that wave the proteins, or antigens, of foreign antigenic invaders like flags. These teach the immune system's T cells to recognize the invading antigens and kill them.

"But dendritic cells vary in quality and number from patient to patient," Oelke pointed out. "Many of them simply can't provide the number of dendritic cells needed to get a vaccine that will work. When a severe infection takes hold, there's a limit to how many dendritic cells the patient can muster."

Oelke, a research immunologist/pathologist at the Johns Hopkins School of Medicine in Baltimore, is first author of an article in Nature Medicine, published online April 21, 2003. Its title: "Ex vivo induction and expansion of antigen-specific cytotoxic T cells by HLA-Ig-coated artificial antigen-presenting cells." Its senior author is pathologist Jonathan Schneck, also at Hopkins.

"The aAPCs that our team manufactured here," Oelke noted, "made twice as many specific targeted cytotoxic T lymphocytes [CTLs] as using dendritic cells - and we could have made even more. Both aAPCs and dendritic cells," he pointed out, "convert generic immune cells in the blood into targeted CTLs."

"Using artificial antigen-presenting cells" Schneck recounted, "we converted run-of-the-mill immune cells into a horde of specific, targeted, invader-fighting machines. Our ability to make vast quantities of these antigen-specific immune cells," he continued, "provides an off-the-shelf technique for creating these man-made cells."

Suctioning White Cells Can Cost Stress, Expense

"To generate enough dendritic cells from, say, a melanoma patient," Oelke told BioWorld Today, "you need to do a leukapheresis [removing those white cells by suction from the circulation]. This procedure, which can last many hours, may cause the patient too much stress," he observed. "Another aspect is that it's extremely expensive to generate dendritic cells, because the patient needs a huge amount of cytokines, and these costs can reach numbers up to $30,000 per patient. That depends on how many dendritic cells can be generated from one leukapheresis.

"The whole idea of these adaptive T-cell transfers," Oelke explained, "is that the immune system itself is not able to get activated against the tumor. Or in another setting, giving immunosuppressed organ transplant patients replacement antibodies to prevent graft rejection.

"Schneck developed soluble MHC [major histocompatibility complex] dimer molecules," Oelke recalled. "Then we both had the idea of using those soluble MHC dimers to create this artificial antigen-presenting cell. That was the goal, which we were able to accomplish. We used magnetic beads, which are about the size of a cell - 4 to 5 microns. On the surface of these beads we immobilized the dimeric molecules. To these we added an anti-CD28 antibody, a co-stimulatory molecule, which was absolutely important and necessary to activate those anticancer and infection-killing T cells. We then exposed the treated beads to infectious antigens from either melanoma or cytomegalovirus.

"So far we haven't done any clinical or preclinical experiments with these artifacts," Oelke recounted. "Our human subjects were healthy donors, plus one melanoma patient. The only way these donors came into our system was that we got blood from them. We isolated the T cells from their circulation and generated dendritic cells. Then we compared the ability of the dendritic cells to the ability of our artificial antigen-presenting cells, to stimulate antigen-specific T cells.

"The blood samples were from either melanoma or cytomegalovirus [CMV] subjects. Then we tested them in an in vitro system, and these dendritic-generated cells were able to recognize and eliminate melanotic tumor cells."

"Using this approach," the Nature Medicine article reported, "we have developed an aAPC that reproducibly stimulates robust CTL expansion, and can be easily manufactured. We show that HLA-Ig-based aAPCs induced and expanded antigen-specific CTLs in vitro to both immunodominant and subdominant epitopes from normal healthy donors - as well as from a patient with extensive metastatic melanoma. The aAPC-mediated stimulation was as effective as, if not better than, dendritic cell stimulation of melanoma's Mart-1 tumor antigen and CMV-induced CTL." The article continued, "We maintained CTLs, using aAPCs, for more than two and a half months of continuous culture, without any detectable loss in specificity or growth rate."

Clinical Trial Planning On Front Burner

"In the future," Oelke observed, "when going into clinical trials, first of all we must think about an adaptive T-cell transfer. That means using these aAPCs in vitro to generate T cells. And after that we will remove these coated beads, and then the T cells will be transferred to the patient.

"We were originally considering preclinical experiments with animals," Oelke recalled, "but on reconsideration if we adopt the T-cell transfer technology, which is already done in humans, we thought animals would not be necessary.

"Meanwhile, we are just about to start a lot of collaborations with different university medical centers to plan clinical trials. We will probably begin these in a setting of malignant melanoma and CMV allogenic bone marrow transplantation. We will then use the cytomegaloviruses to prevent CMV infections in immunosuppressed patients.

"We intend to start these clinical studies as soon as possible," Oelke said, "but I guess we're looking about two years down the road. We need first to generate these aAPCs at a clinical grade," he concluded, "which will take some time and lots of money."

A Hopkins press statement dated April 22 was bannered: "Technique brings immune-based therapies closer to reality." It ended: "Under a licensing agreement between Pharmingen Inc. [of San Diego] and the Johns Hopkins University, Schneck is entitled to a share of royalty received by the university on sale of products related to technology described in this Nature Medicine article."