Cookbook recipes for ex vivo gene therapy overlook the all-purpose Blymphocyte as a vehicle of choice for inserting the desired foreign geneinto the patient's target tissues. So says immunologist Yacov Ron ofNew Jersey's University of Medicine and Dentistry in Piscataway.Ron is principal author of a paper in the current Proceedings of theNational Academy of Sciences (PNAS) titled: "A murine model for B-lymphocyte somatic cell gene therapy."In it, he makes the point that B-cells are reputed solely for their mainrole in the immune system _ making antibodies to order. "Theircapacity to serve as antigen-presenting cells," he points out, as well asother unsung properties, remain neglected.Ron counts the ways in which B lymphocytes might be recruited forother therapeutic missions in the body:* as antigen-presenting cells, to trigger immune counter-attacks againsttumors and viruses;* as autoantigen presenters, to treat autoimmune diseases, by down-regulating the body's immune response, and thus inducing tolerance;* as expression vectors, to get around the drawbacks of current ex vivogene transfer methods. These put insertion constructs into long-term invitro expansion, and/or drug selection, before introducing them in vivo.Ron used mice as demonstrator models for his B-cell gene therapystrategy. "Its novelty," he told BioWorld Today, "is that we can infectlymphocytes with genes and get a very high level of expression,without selection, which most gene therapists have difficultyachieving."Their basic problem, he explained, is relying on gene infectioninvolving selection, which requires weeks in culture. His Blymphocytes, on the contrary, reside in vitro "for the shortest possibletime, around 40 hours, so they don't get damaged." The conventionallengthier duration, he pointed out, is required by the need for aselection method to single out the cells successfully infected by thegene of interest.Selection itself is made necessary, Ron said, by the relatively lowefficiency of gene transfer. This because long-term culture damages theinfected cells so that most of them fail to home in on the tissues atwhich they are aimed. "They simply get signaled to be destroyed in theliver. Why they don't home," he added, "nobody knows."The marker gene he chose to demonstrate the B-cell strategy is thesame adenosine deaminase (ADA)-encoding sequence employed justfour years ago this month to treat a child with severe combinedimmunodeficiency disease (SCID). It was the first clinical gene therapytrial in history. (See BioWorld Today, Sept. 16, 1993, p. 3.)Ron and his co-authors infected B cells with the ADA gene, andadoptively transferred these into SCID mice. Three months later, theanimals' spleens were still continuously expressing high ADA levelsfrom one to three copies of the gene."We think," he observed, "that at least 80 percent of the cells wereinfected, perhaps 90 percent. So it really is very efficient."A separate point in favor of mature primary B lymphocytes," Ronemphasized, is "that if you infect very early progenitor cells, such asbone-marrow stem cells, or even lineage-committed stem cells, oftenyou have a problem expressing the exogenous gene you put in."Gene therapists, Ron suggests, may find his "relatively inexpensive" B-cell animal model useful in pre-testing their own clinical protocols forsomatic gene therapy.Meanwhile, he and his co-workers have begun to focus exclusively onexploring the ability of mature B lymphocytes to manipulate theimmune system down, not up. "If you can get the B cell to express itsantigen in a way that will induce immune unresponsiveness rather thanresponsiveness," he said, "that would be very important in autoimmunediseases, such as multiple sclerosis (MS)."The long-term goal of his lab, he added, "is to down-regulate immuneresponses therapeutically, concentrating on MS." For this, their modelwill be mice adoptively transferred with B cells expressing theautoimmune antigen that triggers -- experimental allergicencephalomyelitis (EAE) -- which approximates the presumed causeof MS."Very soon," Ron said, "we will try, in mice, our first cloned B-cellexpression vectors that contain the autoimmune EAE target gene. If itworks the first time, great. If not we have a lot of other vectors to cloneand try."Within six months, he estimates, "we'll have the first in vivo biologicaldata."One of the three gene therapists who pioneered that first successfulhuman trial four years ago at the National Institutes of Health, isKenneth Culver, now executive director of the Human Gene ResearchInstitute, in Des Moines, Iowa. He told BioWorld Today that the NewJersey team's "delivery vehicle is very important, and that hopefullythe use of this murine model will provide insights into how we mightuse B cells for the treatment of cancer, infectious diseases andautoimmunity -- the areas in which it's likely to be applicable." Culveradded, "If there are some other incremental advances, particularly inthe use of human B cells that can be grafted into the SCID mice, thatwill be an important step forward." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.