At the cellular level, engineered self-destruction would be a boon toscientists fighting autoimmune diseases.
Cells undergo apoptosis _ programmed demise _ when they're nolonger fit, or needed, to survive in the body. T lymphocytes andantibodies are the twin indispensable fighting arms of the immunesystem, except when they run amok and defect, to wreakautoimmunity.
Thus, in multiple sclerosis, certain turncoat T cells attack the myelinprotein that sheaths the nervous system's neurons. In rheumatoidarthritis, clusters of autoantibodies incite the complement system tocreate inflammation in the joints.
In diabetes mellitus, perhaps the widest-spread, most devastating ofall autoimmune diseases, rogue T cells destroy the insulin-secretingbeta cells in the islets of Langerhans, which inhabit the pancreas.Some 700,000 young Americans suffer from autoimmune, insulin-dependent, Type 1 diabetes. (See BioWorld Today, May 31, 1996, p.1.)
Those islets behave like miniature organs in the pancreas. Astransplant surgeons learned in recent decades to replace ailingkidneys, hearts, lungs, liver _ even pancreata _ with donor organgrafts, they also tried to engraft healthy islets in place of thoseravaged by diabetes.
So far, the score is one to nothing in favor of the cells over thesurgeons. The immune system continues to reject transplanted donorislets, despite occasional experimental successes in mice.
In days of old, a knight feeling imminent peril of death at the swordof his adversary could dart into a church and claim sanctuary. Themammalian body includes a handful of such immunologicallyprivileged sites, notably, the cornea, the testes, the uterus and thebrain.
Rendering Islets Immune To Immunity
Transplant surgeon Henry Lau, at Johns Hopkins University inBaltimore, and hematologist Christian Stoeckert Jr. at Philadelphia'sChildren's Hospital, conceived a way to create such a T-cell-proofsafe haven in the body.
To provide islets with sanctuary from autoimmune attack, theresearchers co-transplanted these micro-organs with muscle cells(myocytes) genetically engineered to express a receptor proteincalled FasLigand (FasL). Its target, a newly discovered moleculecalled Fas (a.k.a. CD95), is Doctor Death of the immune system.Expressed on the surface of T lymphocytes, it induces these cells tocommit apoptosis. That makes it a key component of immuneprivilege.
Fas receives this apoptotic death message from its ligand, FasL, andpasses it on to the T cell's interior for execution. FasL's presence oncorneal, testicular, uterine and brain cells is what makes themimmune privileged sites.
A report of how Lau and Stoeckert put this concept into executionappears in the current issue of Science, dated July 5, 1996. It bearsthe title: "Prevention of islet allograft rejection with engineeredmyoblasts expressing FasL in mice."
Those mice are models of human diabetes, rendered insulin-dependent by a dose of streptozotocin, a potent tumor antimetabolitethat kills off their islets of Langerhans.
The team transfected myoblast cells from the thigh muscles ofgenetically simpatico mice with cDNA sequences encoding themurine FasL receptor, and "wrapped" each one around some 300murine islet cells. "That piece of work, the recombinant Fas gene,"Lau told BioWorld Today, "came from my colleague, AdrianoFontana, at the University of Zurich."
Lau and his co-authors at Children's Hospital then injected theirmuscular plasmid delivery package under the renal capsule, whichsheaths the kidney. In this harbor of refuge, the islets' beta cells dulystarted secreting insulin in response to rising glucose in the blood.
Cell, Mouse Survival: What It's All About
Thus ensconced away from graft rejection, and presumably driving Tcells that infiltrate the vicinity to self-destruction, the recombinantFasL kept 31 diabetic mice alive, healthy, active and exerting normalglucose control for periods ranging from 15 to over 100 days.Meanwhile, 16 control mice given dummy injections survived onaverage only 10 days before succumbing to insulin deprivation.
This survival time was dose-dependent. Transplanting 10,000myoblasts lengthened life of the graft from 10 to 26 days; two millionmuscle cells upped the average to more than 84 days.
Once past those promising survival times, the cells sufferedimmunorejection, stopped expressing FasL; the grafts sufferedrejection, and the animals died. "Thus," observed a Science editorialthat accompanied Lau's paper, "before any studies begin in humans,biologists must find a way to ensure long-term FasL expression.What's more," the commentary continued, "the findings must beconfirmed in mice with naturally occurring, rather than chemicallyinduced, diabetes."
"Before we even talk about attempting this approach in humanpatients," Lau emphasized, it needs to be repeated by other people toconfirm what we have done. This whole issue needs to be examinedin the context of an autoimmune diabetes, which is what Type 1 is inhumans. So it needs to be repeated in autoimmune mice." He added,"We're starting that work right now, here at Hopkins."
Lau made the point that, "The use of muscle in this context has a lotof applications besides islet cells. One can imagine transplantingother endocrine tissues, and perhaps even as a means of in vivoimmunomodulation, by expressing different molecules on musclecells." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.