Ever since monoclonal antibodies came on the scene justtwo decades ago, a stubborn hang-up has hampered theiruse in therapy. It goes by the name of the HAMA _human anti-mouse antibody _ reaction.

For a long time after their invention in 1975, the onlysource of monoclonals was the murine immune system.So their use as silver bullets, for example to guide toxicpayloads to tumor targets, ran afoul of the humanpatient's immune system, which reared up and laid lowthe therapeutic monoclonal antibody, by rejecting itsmouse-protein component.

Since those early days, of course, genetic engineeringcontinues to construct ever-improved breeds of"humanized" monoclonals, to overcome, undercut orsidestep HAMA.

Now, human nature is striking back _ at the viral level.

Adenovirus, acclaimed as the ideal vector for ferryingdesirable genes into somatic target cells to correctinherited diseases, is showing its cloven hoof andAchilles heel. It turns out that this virus, though shorn ofits virulence genes, remains susceptible to both arms ofthe mammalian immune system, humoral and cellular.

The former's B cell-generated antibodies pounce onantigens in the virion's coat; the latter dispatch killer Tcells to attack the target cells receiving the precioustransgenes.

So far, this potential backlash "is particularlyproblematic," writes gene therapist James Wilson, "in thetreatment of chronic diseases such as cystic fibrosis thatwill require repeated therapies to obtain life-long geneticreconstitution."

Wilson, who is at the University of Pennsylvania's genetherapy institute in Philadelphia, has National Institutes ofHealth approval to administer an adenovirus vectorconveying the cystic fibrosis transmembrane conductanceregulator gene by inhalation into the lungs of 24 cysticfibrosis patients. After treating the first eight patients, hesuspended this trial, while attempting to doctor the vectorto permit more stable, and repeat, insufflation of the viruswithout fear of an adverse immune reaction.

Now, after about nine months of applied research, he canreport preclinical progress. This month's NatureMedicine carries Wilson's paper titled, "Recombinant IL-12 prevents formation of blocking IgA antibodies torecombinant adenovirus and allows repeated gene therapyto mouse lung."

Body's Immune Defenses Block Its Own Gene Therapy

The adenoviral surface proteins whose antigens were"seen" and attacked by the antibodies, Wilson toldBioWorld Today, "are the same proteins on theadenovirus that enable it to be recognized by the targetcell and internalized. So it blocks that effect," preventingthe vector from delivering its cystic fibrosis therapeuticgenes to their pulmonary target cystic fibrosis cell.

So much for the humoral (antibody-generating) immuneresponse.

"The cellular immune response," Wilson continued, "inwhich T cells target the virally infected target cells andsomehow destroy them, results because the viral genomeexpresses viral proteins, which the body views as infectedcells, to be disposed of."

He is confident that "with time we're going to be able toget around that by re-engineering the vector so it's notleaky _ in terms of expressing viral proteins. But it's thevirion itself," he pointed out, "that's going to beproblematic. The way I view it is that in animals there'sclearly a problem with adenovirus. Whether or not thatwill be the case in humans, requires our evaluation. Idon't think there's any reason to expect it wouldn't."

Wilson explained: "It's kind of naive to think that if yougive large, complex, proteinaceous particles repeatedly,the body isn't going to mount an immune response."

In their mice, the researchers determined that theneutralizing antibodies that hit the adenovirus antigensarose from an immune response by the body'simmunoglobulin A (IgA). This B cell process in turndepended on T helper cells of the TH2 persuasion, asdistinct from the other brand of helpers, TH1. In the balletof immune-system reactions, interleukin-12 (IL-12)activates TH1 helper T cells to secrete interferon-gamma(IFN-g), which inhibits TH2.

Thus, Wilson and his co-authors reasoned, co-instillingeither cytokine, IL-12 or IFN-g, or CD4 antibody, alongwith the adenovirus vector plus a model gene therapytransgene, into the airways of their mice would "diminishthe activation of TH2 cells and formation of neutralizingantibodies, allowing for efficient readministration ofrecombinant virus."

Their proof-of principle experiments described in NatureMedicine bore out this concept, and have since ledWilson and his group to discover and develop other moreefficient immune modulators to protect adenovirusvectors during a second round of gene therapy. "They'reagents that interfere with the interaction of T cells andother T cells, or B cells," he observed.

"We have evaluated other approaches for blocking theimmune response," Wilson said, "other than cytokinesand CD4 antibodies, using reagents under development inthe biotech and pharmaceutical industries. We haveshown the same results with several other agents at thispoint, and we may have some papers coming out on thatsoon."

Moreover, the team has extended its studies "from cysticfibrosis lungs into liver, for gene therapy of hemophiliaand familial hypercholesterolemia, on which we're nowworking, with the same results. Clearly," Wilsonobserved, "the same problems exist in the liver, and ourresults _ in terms of other immune-modulating drugs _is that they have similar if not identical effects."

Cystic Fibrosis Trials: From Stop To Go By Year's End

Meanwhile, as a prelude to re-starting the cystic fibrosisclinical trials, Wilson said, "We have undertaken a veryextensive program in modifying the adenovirus vector.By now we must have a dozen different versions of thevirus, which represent forms that have different deletionsand other mutations."

He added, "We're now in a position to reinitiate thecystic fibrosis clinical trials, which were stopped until wehad completed this comparative analysis."

At present, Wilson said, "We're now bringing it towardtoxicology and manufacturing, and hope to restart ourtrials before the first of the year, with a substantiallyrevised vector."

In those remaining 14 patients, "The next phase will be:Let's continue the trial with and without an immunemodulator, and then look at repeated administration."

The main benefit he sees in immuno-modulation toprotect the adenovirus vector, Wilson concluded, "is co-administering it with the vector, not chronically givingthe drug afterward, as is done for organ-transplantpatients. So if you give the gene therapy vector packageonce a year, you give the drug once a year." n

-- David N. Leff Science Editor

(c) 1997 American Health Consultants. All rights reserved.