Biomedical Engineering Reference
In-Depth Information
inluence.not.only.the.magnitude.of.response,.but.also.the.type.of.immune.response..(See.
Kim.and.Weiner
14
.for.a.more.detailed.analysis.)
14.2.3 Early Evaluation of DNA Vaccines in Human
In.mice.and.smaller.organisms,.DNA.vaccines.are.highly.effective.and.generate.strong.and.
often.protective.immune.responses.against.various.pathogens..Consequently,.much.effort.
has.been.made.in.expanding.the.use.of.DNA.vaccines.in.humans..The.earliest.DNA.vaccine.
trials.in.humans.investigated.the.safety.of.intramuscularly.injecting.patients.with.plasmid.
DNA,.and.these.studies.all.suggested.that.plasmids.were.well.tolerated.with.no.measurable.
anti-DNA.immune.response.
15,16
.However,.unlike.the.magnitude.of.immune.response.that.
was.generated.in.mice,.the.magnitude.in.humans.was.minimal.at.best..In.one.study,.HIV-
negative.volunteers.were.immunized.with.an.HIV.
Env/Rev
DNA.vaccine,.and.the.authors.
reported. that. patients. receiving. the. highest. dose. did. exhibit. vaccine-induced. immune.
responses.. However,. the. effects. were. often. transient,. and. the. magnitude. of. the. CD8
+
. T.
cell.response.was.negligible,.with.fewer.than.35.spot-forming.units.(SFU).in.an.interferon.
(IFN)-γ. ELISpot. assay.
15
. These. early. studies. all. suggested. that. DNA. immunization. may.
stimulate.antigen-speciic.immune.responses.in.humans,.but.the.magnitude.of.response.
was.far.from.that.of.attenuated.vaccines,.and.deinitely.not.within.range.to.provide.protec-
tion,.suggesting.the.need.for.more.potent.and.effective.second-generation.vaccines.
Although. many. factors. could. affect. the. eficacy. of. DNA. vaccines. in. humans,. the. pri-
mary.reason.appears.to.be.ineficient.transfection.of.muscles.cells..Upon.DNA.injection.
into.the.muscle,.the.plasmids.are.rapidly.taken.up.by.the.muscle.cells,.which.then.express.
the.antigens..Although.the.plasmids.may.also.directly.transfect.APCs,.this.mechanism.is.
not.the.primary.one.for.priming.the.immune.system..Several.pieces.of.evidence.support.
this. notion;. irst,. upon. intramuscular. injection,. the. injected. DNA. is. detected. primarily.
at.the.site.of.injection,.and.any.DNA.detected.at.other.sites,.such.as.the.draining.lymph.
nodes,.does.not.confer.gene.expression.in.terms.of.both.mRNA.and.protein.expression.
17
.
Therefore,. it's. more. likely. that. DNA. detected. in. these. regions. is. likely. from. APCs. that.
have. taken. up. dead,. plasmid-containing. cells. or. degraded. extracellular. DNA.. A. more.
cogent.piece.of.evidence.is.that.DNA.plasmids.whose.promoters.are.muscle.speciic.are.
equally.as.effective.as.plasmids.whose.promoters.are.ubiquitously. active.in.stimulating.
the.immune.system.
18
.Therefore,.even.if.plasmids.do.get.transfected.directly.into.APCs,.
the. ability. of. the. APCs. to. actually. express. the. protein. is. not. critical. for. stimulating. the.
immune.system.
So.if.DNA.transfection.of.muscle.cells.is.important,.is.there.a.difference.in.the.mode.or.
eficiency. of. transfection. in. smaller. animals. versus. larger. animals?. Dupuis. et. al.. inves-
tigated. this. exact. question. by. performing. two. distinct. experiments.. First,. the. authors.
altered.the.total.volume.of.the.vaccine.while.maintaining.the.absolute.DNA.amount;.the.
difference.in.volume.would.control.DNA.dispersion.in.the.tissue.following.injection.and.
alter. the. hydrostatic. pressure. caused. by. the. injection.. This. experiment. would. address.
whether.vaccine.volume.could.play.a.role.in.determining.vaccine.eficiency,.considering.
the.volume.used.in.mice.could.never.be.scaled.up.to.be.used.in.humans.and.macaques..
Second,. the. authors. also. electroporated. the. tissue. following. injection. to. increase. DNA.
uptake..Interestingly,.these.authors.demonstrated.that.injecting.DNA.at.a.lower.concen-
tration,. or. higher. volume,. increased. the. eficiency. of. plasmid. transfection. and. vaccine-
induced.immune.response.
17,19
.The.exact.mechanism(s).for.increased.DNA.uptake.by.the.
muscle.cells.is.not.known,.and.the.importance.of.this.phenomenon.in.the.context.of.DNA.
immunization. in. humans.remains. unknown..Confounding. to. the. volume. hypothesis. is.
