Clinical development of therapies for septic shock has been aminefield for the biotechnology industry. This bleak landscape islittered with the remains of failed clinical trials and crippledcompanies. The difficulties encountered in developing treatments forsepsis primarily result from the complexity of both the clinicalcondition and its underlying biological causes.
One of the major molecular culprits in sepsis is tumor necrosis factor(TNF). TNF has multiple biological effects, most of which are partof a cascade of reactions that cause septic shock. The dominanteffects of TNF include cell death, generalized tissue necrosis, controlof antiviral activity and cytokine production. TNF elicits theseeffects through two distinct cell surface receptors, TNF-R1 and TNF-R2.
Most biological responses to TNF occur when it binds to its TNF-RIreceptor. These effects include the systemic reaction to low doses oflipopolysaccharide (LPS) that characterizes the septic shocksyndrome. In contrast, the role of TNF-R2 is much less wellunderstood. This second receptor supposedly mediates T celldevelopment, and proliferation of cytotoxic T cells. Scientists have afar better understanding of TNF-RL1 function than of TNF-R2.
One who has light to shed on R2 is molecular immunologist MarkMoore, a senior scientist at South San Francisco-based GenentechInc. He is last author of an article titled "Decreased sensitivity totumor-necrosis factor but normal T- cell development in TNF-receptor-2-deficient mice." Appearing in last week's Nature, datedDec. 8, it goes some distance towards increasing knowledge aboutthe function of TNF-R2.
Moore and his co-authors at Genentech and Washington UniversitySchool of Medicine show that TNF-R2 plays a major role in TNF-induced tissue necrosis In contrast to previously held belief, thisreceptor does not seem to play a significant role in normal T-celldevelopment and activity, but it does mediate TNF-induced T-celldeath.
These researchers dissected out TNF-R2's effects by generatingmouse strains that lacked a functional version of this receptor. Theyconstructed these "knockout" rodents by what is fast becomingstandard operating procedures for gene transfer:
First, insert a neomycin (nep)-resistance gene into the second exonof the 9-kilobase TNF-R2 genomic clone, thus inactivating the gene.
Second, introduce this construct into mouse embryonic stem (ES)cell lines.
Third, select for ES cell colonies containing the TNF-R2 construct.
Fourth, microinject these into blastocysts, to generate transgenicmice.
Fifth, interbreed the transgenic strains that show germlinetransmission of this construct, to produce a homozygous strain thatlacks functioning TNF-R2.
As BioWorld Today was going to press, Genentech's Moore wasunavailable for comment. But the results reported in Nature do showthat TNF-R2 is not responsible for the toxic systemic effects of LPSor TNF that afflicted the R2-minus mice.
However, the dramatic reduction in tissue necrosis seen in the R2-deficient animals does demonstrate a decreased sensitivity to TNF inthe absence of this receptor. It suggests that the two receptors aresynergistic in transducing TNF signals to the cellular interior.
Also, as mice deficient in either receptor type proved still sensitive tohigh doses of LPS, both receptors appear to make a contribution toLPS-induced septic shock. Alternatively, TNF-independentproduction of other cytokines contributed greatly to toxicity duringsepsis. Generations of transgenic knockout mice that are deficient inboth R1 and R2 should further clarify their interactions and roles inthis frequently fatal infection, the Nature paper observed.
With the potential market for septic-shock treatments estimated atover $500 million per year, the need to understand sepsis as aclinical condition is great, and in prospect materially rewarding.Because it results from a rapidly and dramatically occurring cascadeof multiple, interacting biological processes, septic shock is difficultto treat. n
-- Chester Bisbee Special To BioWorld Today
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