The En­emy of My En­emy is My Friend

A Mi­cro­bial So­lu­tion to The Em­peror of all Mal­adies

by Elie J. Diner

I love a good un­der­dog story, and I'm pretty sure I'm not alone. Just look at the pop­u­lar­ity of sports movies like Rudy, Rocky, Hoosiers and this blogger's fa­vorite, the Mighty Ducks. What is it about these sto­ries that cap­ture our col­lec­tive imag­i­na­tions? There is some­thing al­lur­ing about watch­ing a group of seem­ingly av­er­age in­di­vid­u­als achieve great­ness against the odds. And as bi­ol­o­gists who think about nat­ural se­lec­tion on a daily ba­sis, where the lit­tle guy typ­i­cally gets eaten by the big­ger guy, it is fun to see the roles re­versed.

Fig­ure 1. Bone sur­geon William Bradley Co­ley

Fol­low­ing the State of the Union Ad­dress by Pres­i­dent Obama and his de­c­la­ra­tion "to cure can­cer once and for all," a re­cent post on this blog pon­dered the fol­low­ing: How can mi­crobes con­tribute to this ef­fort of find­ing a cure for can­cer? This seems to be a true un­der­dog sit­u­a­tion con­sid­er­ing the size dif­fer­ence be­tween a bac­te­r­ial cell (~1 µm³) and a mam­malian can­cer cell (~2,000 µm³). Given how many of these can­cer­ous cells could make up a tu­mor, this head-to-head bat­tle seems to have a pre­de­ter­mined win­ner; the lit­tle guys don't stand a chance. But as we know from the pro­to­typ­i­cal un­der­dog, David, and his foe Go­liath, size is not al­ways the cru­cial fac­tor. And as read­ers of this blog are aware, bac­te­ria have quite a few tricks up their mi­cro­scopic sleeves. For now, I will leave other mi­crobes (viruses and sin­gle-celled eu­kary­otes) out of this dis­cus­sion, but do not be fooled, even the al­gae can con­tribute to the can­cer fight­ing cause. Be­low, I di­vide up our ex­am­ples of can­cer fight­ing bac­te­ria into first, their use as a can­cer vac­cine and sec­ond, the abil­ity of some bac­te­r­ial species to "sniff" out the most elu­sive of tu­mors.

Get­ting by with a lit­tle help from my in­flam­ma­tory friends

Dur­ing the course of in­fect­ing an un­lucky hu­man, some path­o­genic bac­te­ria evade de­tec­tion by the mam­malian im­mune sys­tem, re­spon­si­ble for rec­og­niz­ing and elim­i­nat­ing any­thing for­eign. In do­ing so, these pathogens setup their home in their fa­vorite tis­sue and can make our lives pretty un­pleas­ant. Most bac­te­ria are not so crafty and tend to present a plethora of mol­e­cules (of­ten called PAMPs) that can send the im­mune sys­tem into red alert. This alert is of­ten lit­eral, lead­ing to in­creased blood flow (red­den­ing), swelling of the in­fected tis­sue, a gen­eral in­crease in body tem­per­a­ture (the hall­marks of in­flam­ma­tion). Plus, tis­sues be­come in­fil­trated by im­mune cells that re­lease in­flam­ma­tory sig­nals called cy­tokines, im­por­tant ac­tors in the elim­i­na­tion of all things bac­te­r­ial. So, what if this dra­matic im­mune re­sponse could be har­nessed and fo­cused on the elim­i­na­tion of can­cer­ous cells in­stead of bac­te­ria?

There has been sig­nif­i­cant ef­fort in re­cent years on this front, spawn­ing a new field called can­cer im­munother­apy. This has proven dif­fi­cult, as the im­mune sys­tem goes to great lengths to pre­vent the recog­ni­tion of it­self, and can­cer­ous cells con­tain many of the same recog­ni­tion el­e­ments as nor­mal cells. At best, this makes can­cer cells only weakly im­muno­genic.

En­ter William Co­ley (Fig­ure 1), a prac­tic­ing bone sur­geon in the mid-1800s and one of the first to dis­cover that one could har­ness the im­mune re­sponse and di­rect it at can­cer. Dev­as­tated by the loss of his first pa­tient, Co­ley scoured old med­ical records, look­ing for pa­tients who had serendip­i­tously sur­vived sar­co­mas (say it three times fast!). Co­ley found what he was look­ing for in a pa­tient that had been op­er­ated on re­peat­edly in an at­tempt to re­move a sar­coma oc­cu­py­ing his cheek. As you may imag­ine, 19^th cen­tury sur­gi­cal tech­nique was not as asep­tic as it is to­day, and the pa­tient came down with a Strep­to­coc­cus pyo­genes in­fec­tion fol­low­ing one of his many surg­eries. Be­ing a pre-an­tibi­otic era, there was not much to do for the pa­tient. Yet, fol­low­ing a sharp fever, not only did the pa­tient re­cover from the bac­te­r­ial in­fec­tion but his sar­coma dis­ap­peared. Co­ley was struck by this mirac­u­lous re­cov­ery and he tracked down the pa­tient to con­firm the story, not­ing the pa­tient was still tu­mor free. Co­ley be­gan treat­ing pa­tients with in­op­er­a­ble sar­co­mas us­ing live S. pyo­genes, in­jected at the tu­mor site. This was a risky treat­ment plan on Coley's part, yet he dis­cov­ered that fever was a crit­i­cal step in tu­mor re­gres­sion. Af­ter fur­ther re­fin­ing his meth­ods, Co­ley be­gan us­ing a cock­tail of heat-killed S. pyo­genes and Ser­ra­tia marcescens to in­duce fever more re­pro­ducibly and lessen the risk of caus­ing an ac­tive bac­te­r­ial in­fec­tion. This vac­cine came to be known as 'Coley's toxin' (a bit of a mis­nomer that stemmed from the idea that a spe­cific bac­te­r­ial toxin caused tu­mor re­gres­sion) and through an in­jec­tion reg­i­men he de­vel­oped, not only cured sar­co­mas, but also melanomas, car­ci­no­mas, lym­phomas, and myelo­mas.

How could a can­cer panacea like this not be in use to­day!? Many doc­tors viewed Coley's re­sults with skep­ti­cism and some were un­able to re­pro­duce them. This con­tro­versy and the grow­ing pop­u­lar­ity of ra­di­a­tion ther­apy at the turn of the cen­tury con­tributed to Coley's vac­cine falling from fa­vor. While there have been many mod­ern day stud­ies, the mech­a­nism by which Coley's toxin leads to tu­mor re­gres­sion is still not well ex­plained. Many have tried to re­pro­duce the an­ti­cancer re­sponse us­ing a pu­ri­fied bac­te­r­ial fac­tor or cy­tokine to no avail.

Even so, in­ter­est in Coley's vac­cine has seen a re­birth in re­cent years with the found­ing of a San Diego biotech com­pany, at­tempt­ing to im­prove Coley's vac­cine and turn it into a com­mer­cially avail­able prod­uct. The com­pany, De­coy Biosys­tems, was re­cently fea­tured as one of the top 12 San Diego biotech star­tups to watch in 2016 by Xcon­omy. So, per­haps there will be vin­di­ca­tion for Co­ley and his can­cer killing bac­te­r­ial vac­cine.

Mi casa es su casa

The home of a tu­mor can be a mess: chaotic blood flow can cre­ate re­gions lack­ing oxy­gen or a car­bon source. Thus, cells within a tu­mor can die, spilling their con­tents into the ex­tra­cel­lu­lar mi­lieu. Ul­ti­mately, this re­sults in a jum­bled mass of can­cer­ous cells in var­i­ous states of death, in­ac­tiv­ity, or vig­or­ous growth. This ar­chi­tec­ture also makes many tu­mors tough to reach by im­mune cells or small mol­e­cule an­ti­cancer drugs.
 

Fig­ure 2. (A) In a per­fect world, an en­gi­neered can­cer ther­apy can tar­get tu­mors through self-propul­sion and de­liver drugs. (B) Bac­te­ria have these ca­pa­bil­i­ties and © the num­ber of pa­pers pub­lished on us­ing them for can­cer ther­apy is on the rise. Source

While all of these things are cons for the pas­sive dif­fu­sion of a small mol­e­cule, they are pros for bac­te­ria that can not only sense the pres­ence of food­stuff, e.g., amino acids and sug­ars, but also swim chemo­tac­ti­cally to­wards it. Such bac­te­ria (e.g., Sal­mo­nella en­ter­ica ser­rotype ty­phimurium) can nav­i­gate the com­plex vas­cu­la­ture sur­round­ing a tu­mor and pen­e­trate deep within its tis­sue. Un­like the meth­ods de­scribed by Co­ley, which re­quired lo­cal in­jec­tion of bac­te­ria into a tu­mor, sev­eral groups have shown that many bac­te­ria, in­clud­ing S. ty­phimurium (let me call it by its short­ened name), can be ad­min­is­tered sys­tem­i­cally and will ac­cu­mu­late 1000-fold more in tu­mors than in nor­mal tis­sues. Specif­i­cally, S. ty­phimurium chemo­taxes to­wards ser­ine, as­par­tate, and ri­bose. In ad­di­tion, S. ty­phimurium aux­otrophic for leucine and argi­nine has a greater speci­ficity for lo­cal­iz­ing to many types of tu­mors. Read­ers of STC may have al­ready been aware of the work of Abe Eisen­stark (pro­filed here), who has worked for some time on the abil­ity of Sal­mo­nella to lo­cal­ize to prostate tu­mors. Ob­lig­ate anaer­o­bic bac­te­ria, like Clostrid­ium or Bi­fi­dobac­terium, also lo­cal­ize specif­i­cally to tu­mors when ad­min­is­tered sys­tem­i­cally, be­ing that they grow specif­i­cally in the anaer­o­bic en­vi­ron­ment deep within a tu­mor.

So, what can be done with these tu­mor-sniff­ing mi­crobes? Sev­eral groups have shown that the nat­ural abil­ity of Clostrid­ium and Sal­mo­nella to stim­u­late an im­mune re­sponse can lead to tu­mor re­gres­sion. In ad­di­tion, the abil­ity to ge­net­i­cally en­gi­neer bac­te­ria al­lows for the ex­pres­sion of ther­a­peu­tic pro­teins at the tu­mor site by mi­crobes that have in­fil­trated a tu­mor (Fig­ure 2). This en­ables a whole new class of treat­ment op­tions and the unique abil­ity to de­liver drugs specif­i­cally to a tu­mor.

Be­ware of the glow­ing urine

The abil­ity of bac­te­ria to lo­cal­ize to tu­mors not only al­lows them to de­liver ther­a­peu­tics, but also act as a sen­tinel for tu­mor de­tec­tion. Re­cently, a liver can­cer de­tec­tion method was de­signed us­ing the Es­cherichia coli Nissle 1917 strain, en­gi­neered to ex­press high lev­els of LacZ (beta-galac­tosi­dase). This strain does not re­quire lo­cal or sys­temic, but can be de­liv­ered orally. It ac­cu­mu­lates in liver tu­mors or their de­rived metas­tases, for which meth­ods of de­tec­tion are se­verely lack­ing. In­jec­tion of a mol­e­cule called Lu­Gal (con­ju­gate of lu­ciferin and galac­tose) in­tra­venously al­lows it to pass through the liver and be cleaved by LacZ, ex­pressed by E. coli Nissle that is grow­ing specif­i­cally in liver tu­mors. Fol­low­ing cleav­age, the re­leased lu­ciferin trav­els to the kid­neys where it is cleared in the urine. A sim­ple lu­mi­nes­cence test of the urine then pro­vides a sen­si­tive di­ag­nos­tic for the pres­ence of liver tu­mors (Fig­ure 3).

Fig­ure 3. Di­a­gram for the use of bac­te­ria as a can­cer di­ag­nos­tic. Source

The re­cent push for a cure on can­cer has been called a "moon­shot," the goal of which is to have a ther­a­peu­tic for the ef­fec­tive treat­ment of can­cer as early as 2020. Given the can­cer-fight­ing and can­cer-de­tect­ing tal­ents of the bac­te­ria de­scribed here, it seems clear that these lit­tle crit­ters could play a huge role in the cure for can­cer. The role of the mi­cro­biota in tu­mori­ge­n­e­sis and even the ef­fi­cacy of chemother­a­peu­tic agents is cur­rently emerg­ing as an­other im­por­tant as­pect of bac­te­ria and can­cer. So, with bac­te­ria on your side, this "moon­shot" seems more like a slam dunk!

 

Elie Diner, here with Rachel Diner, is a post­doc­toral fel­low in the Romes­berg lab at The Scripps Re­search In­sti­tute in La Jolla, CA. He is in­ter­ested in syn­thetic bi­ol­ogy, ex­pan­sion of the ge­netic code and micro­biology. In ad­di­tion to sci­ence, he en­joys surf­ing, camp­ing and spend­ing time out­side.