Viruses that In­fect Par­a­sites that In­fect Us

The Ma­tryoshka Dolls of Hu­man Pathogens

by Jamie Schafer

Source

We're all too fa­mil­iar with the viruses that can in­fect us, from the com­mon cold to yel­low fever virus to the en­doge­nous retro­viruses that make up a chunk of our genome. Many of us are also ac­quainted with par­a­sites, such as tape worms or Gi­a­r­dia, that like to set up camp in the hu­man body. But the world of par­a­sites and viruses does not end there. Many par­a­sites or en­dosym­bionts can be in­fected with viruses. A clas­sic ex­am­ple is Para­me­cium, which can har­bor an en­dosym­bi­otic bac­terium, Caed­ibac­ter, which in turn car­ries phages in­volved in mak­ing a toxin. But from the hu­man point of view, things start to get par­tic­u­larly in­ter­est­ing when we con­sider the viruses that in­fect par­a­sites of hu­mans and how those vi­ral in­fec­tions — in­side of a par­a­site in­side of a per­son, some­what like a Ma­tryoshka nest­ing doll — may mod­u­late the parasite's in­ter­ac­tion with its hu­man host. Sev­eral pro­to­zoan par­a­sites, in­clud­ing Leish­ma­nia and Tri­chomonas, are in turn par­a­sitized by viruses, namely leish­ma­nia RNA virus‑1 (LRV‑1) and Tri­chomonasviruses, re­spec­tively. A com­mon­al­ity among such viruses is that they all have dsRNA genomes. It re­mains to be seen whether they also share the prac­tice of ex­ac­er­bat­ing par­a­sitic in­fec­tion of the host as was re­cently re­ported for LRV‑1 in­fec­tion of Leish­ma­nia guya­nen­sis.

A Phle­boto­mus pa­p­atasi sand­fly in the process of in­gesting its blood­meal, which is vis­i­ble through its dis­tended trans­par­ent ab­domen. Sand­flies such as this P. pa­p­atasi are vec­tors for the spread of L. guya­nen­sis. Source

L. guya­nen­sis is trans­mit­ted by a sand­fly vec­tor and, once passed on to the hu­man host, pref­er­en­tially in­fects macrophages, lo­cal­iz­ing to the phagolyso­some within these cells. In­fec­tion by L. guya­nen­sis may cause ei­ther cu­ta­neous leish­ma­ni­a­sis (CL), which ex­hibits as le­sions on the skin, or mu­co­cu­ta­neous leish­ma­ni­a­sis (MCL), a more se­ri­ous con­di­tion that dam­ages much of the na­sopha­ryn­geal cav­ity, some­times even de­stroy­ing the nose of in­fected in­di­vid­u­als. Since MCL de­vel­ops in 5–10% of in­fec­tions with L. guya­nen­sis, fig­ur­ing out what causes this se­vere con­di­tion is of ob­vi­ous in­ter­est.

There have been some in­ves­ti­ga­tions into what might pre­dis­pose a pa­tient to de­velop one con­di­tion or the other but these stud­ies may not pro­vide a com­plete an­swer. MCL clin­i­cal iso­lates con­sis­tently cause MCL in ham­sters sug­gest­ing some­thing is hap­pen­ing on the Leish­ma­nia side too. One of the ma­jor dif­fer­ences be­tween CL and MCL is that a lot more in­flam­ma­tion is in­volved in MCL. A re­cent Sci­ence pa­per by Ives et al. re­ports that in­fec­tion of L. guya­nen­sis by LRV‑1 may play a role in in­creas­ing the in­flam­ma­tory sig­nals pro­duced by in­fected macrophages. The ev­i­dence? LRV‑1 RNA was con­sis­tently de­tected in MCL iso­lates and in L. guya­nen­sis strains that were termed metasta­tic be­cause they con­sis­tently cause MCL in ham­sters. In CL iso­lates and in non-metasta­tic par­a­sites, vi­ral RNA was not de­tected, or was present at much lower lev­els. Fur­ther­more, the ex­pres­sion level of proin­flam­ma­tory genes for the cy­tokines CCL5, CXCL10, and IL‑6 was greater in macrophages in­fected with L. guya­nen­sis con­tain­ing higher lev­els of LRV‑1. Like­wise, the level of these pro­teins was much higher dur­ing in­fec­tions with MCL iso­lates than with CL iso­lates. In both sit­u­a­tions, the in­crease in in­flam­ma­tory me­di­a­tors de­pended on the ex­pres­sion of Toll-like re­cep­tor 3 (TLR3).

Cy­tokine abun­dance was quan­ti­fied af­ter 6 hours of in­fec­tion of macrophages from wild-type C57BL/6 mice and TLR3–/– mice, with virus-in­fected, virus-free, meta­static, or non-metasta­tic par­a­sites. PolyI:C is a con­trol for TLR3-de­pen­dent stim­u­la­tion, whereas LPS is a con­trol for TLR3-in­de­pen­dent stim­u­la­tion. Data re­flect mean ± SD of pro­tein se­cre­tion rel­a­tive to un­stim­u­lated con­trols (n = 2). Source (with mod­i­fied leg­end).

The au­thors make the as­ser­tion — and quite a rea­son­able one — that while L. guya­nen­sis is re­sid­ing within the phagolyso­some, free vi­ral dsRNA may stim­u­late sig­nal­ing through TLR3, a re­cep­tor lo­cated in en­do­somes. Sig­nal­ing through the TLR‑3 path­way ac­ti­vates NF-κB which then pro­motes ex­pres­sion of type I in­ter­fer­ons, lead­ing to ex­pres­sion of many in­flam­ma­tory genes. Un­like your typ­i­cal virus that pro­duces ex­tra­cel­lu­lar in­fec­tious viri­ons, LRV‑1 re­mains within the leish­ma­nias, re­ly­ing on trans­mis­sion to daugh­ter cells dur­ing cell di­vi­sion. Thus any free vi­ral RNA in the phagolyso­some would have been re­leased from dead par­a­sites. There­fore it seems that the in­flam­ma­tion caused by LRV‑1 may not be ad­van­ta­geous to L. guya­nen­sis per se, even though it does cause more se­vere pathol­ogy. Rather it may in fact be an un­for­tu­nate side-ef­fect of a macrophage suc­cess­fully deal­ing with the par­a­sites that in­fect it. To date, this pa­per is the only known in­stance where vi­ral in­fec­tion of a pro­to­zoan par­a­site mod­u­lates its in­ter­ac­tions with its host. The search is on to find oth­ers.

SEM of T. vagi­nalis (T) ad­hered to vagi­nal ep­ithe­lial cells. (a) The ep­ithe­lial mono­layer ex­hibits signs of in­jury (ar­rows), such as re­trac­tion from neigh­bor­ing cells. (b) T. vagi­nalis, which is nor­mally pear-shaped, flat­tens it­self af­ter at­tach­ing to vagi­nal ep­ithe­lial cells, max­i­miz­ing the sur­face area be­tween the par­a­site and the host cell. Doderlein's lac­to­bacilli (B) can also be seen. AF, an­te­rior fla­gella; RF, re­cur­rent fla­gel­lum. Scale bars: (a) 10 μm; (b) 4 μm.. Source

Tri­chomonas vagi­nalis is a com­mon, sex­u­ally trans­mit­ted par­a­site that causes vagini­tis, has been im­pli­cated in pre­ma­ture births, and may also be as­so­ci­ated with de­vel­op­ment of prostate and cer­vi­cal can­cers. While T. vagi­nalis doesn't have a bi­fur­cated pre­sen­ta­tion like L. guya­nen­sis'MCL and CL, it does share L. guya­nen­sis' pen­chant for vi­ral in­fec­tion. In fact, it seems that one virus is not enough for T. vagi­nalis, with some clin­i­cal sam­ples con­tain­ing rep­re­sen­ta­tives of each of the four species of tri­chomonasviruses (TVVs) and one iso­lated cell har­bor­ing three dif­fer­ent viruses. The ef­fect that these vi­ral in­fec­tions have on the path­o­genic­ity of T. vagi­nalis has not yet been de­ter­mined. How­ever, some­thing is known about the ef­fect of the virus upon the par­a­site it­self and this may pro­vide clues re­gard­ing po­ten­tial ram­i­fi­ca­tions for tri­chomo­ni­a­sis. First, TVVs al­ter ex­pres­sion of P270, an im­muno­genic sur­face pro­tein in T. vagi­nalis. Ad­di­tion­ally, TVV in­fec­tion mod­u­lates ex­pres­sion of cys­teine pro­teases, which are known T. vagi­nalis vir­u­lence fac­tors. Since T. vagi­nalis is an ex­tra­cel­lu­lar par­a­site, it would not in­ter­act with TLRs in the phagolyso­some like Leish­ma­nia does. How­ever, it may en­counter these sen­sors, for ex­am­ple, upon be­ing en­gulfed by a macrophage, in which case stim­u­la­tion by TVV dsRNA might in­crease in­flam­ma­tion. This po­ten­tial for ex­ac­er­bated in­flam­ma­tion, along with TVV's mod­u­la­tion of pro­teins im­por­tant for im­muno­genic­ity and vir­u­lence, make it seem less a ques­tion of whether TVV in­fec­tion af­fects the course of tri­chomo­ni­a­sis, and more likely a ques­tion of how.

Leish­ma­nia and Tri­chomonas ob­vi­ously do not rep­re­sent an ex­haus­tive list of mi­cro­bial par­a­sites in­fected by viruses. Gi­a­r­dia lam­blia is home to gi­a­r­diaviruses and En­ta­moeba his­tolyt­ica seem to be har­bor­ing a few of their own. Nor is this no­tion of a virus in­side of a par­a­site in­side of a per­son the sole ex­am­ple of bi­o­log­i­cal ma­tryoshka dolls. Ne­orick­ettsia sen­netsu takes nest­ing to a new level. It in­fects a trema­tode which then in­fects a fish, which when eaten can trans­mit both the trema­tode and the ne­orick­ettsia to whichever hun­gry per­son didn't take the time to cook their fish be­fore eat­ing it.

Much re­mains to be known about how these two par­tic­u­lar viruses af­fect hu­man health, while many viruses within par­a­sites — and per­haps even within viruses them­selves — are likely yet to be iden­ti­fied. We may come to re­al­ize that many par­a­sitic clin­i­cal con­di­tions are like onions: they have lay­ers.

Ref­er­ence

Ives A, Ronet C, Pre­vel F, Ruz­zante G, Fuertes-Mar­raco S, Schutz F, Zang­ger H, Re­vaz-Bre­ton M, Lye LF, Hick­er­son SM, Bev­er­ley SM, Acha-Or­bea H, Launois P, Fasel N, Masina S. (2011). Leish­ma­nia RNA virus con­trols the sever­ity of mu­co­cu­ta­neous leish­ma­ni­a­sis. Sci­ence (New York, N.Y.), 331 (6018), 775–778. PMID 21311023

Jamie is a stu­dent in the Ph.D. pro­gram in Vi­rol­ogy at Har­vard Uni­ver­sity. She stud­ies NK cells in SIV in­fec­tion.