Leishmania donovani Parasites Interact with γ/δ+ Human Peripheral Blood T Cells and Induce Susceptibility to NK Cell-Mediated Lysis

Leishmania donovaniParasites Interact withg/dþ Human PeripheralBlood T Cells and Induce Susceptibility to NK Cell-Mediated Lysis

A. SAHA*, G. CHAKRABARTI*, S. SEN† & S. BANDYOPADHYAY*

* Indian Institute of Chemical Biology, Calcutta, and†Sen Medical Research Centre, Patna, India

(Received 5 May 1999; Accepted in revised form 17 August 1999)

Saha A, Chakrabarti G, Sen S, Bandyopadhyay S.Leishmania donovaniParasites Interact withg/dþ HumanPeripheral Blood T Cells and Induce Susceptibility to NK Cell-Mediated Lysis. Scand J Immunol1999;50:588–595

We recently reported thatLeishmania donovaniinfect the human T-cell linein vitro. To examine whetherprimary human T cells could be infected by this parasite, a direct interaction of the peripheral blood T cellswith L. donovaniwas examined. The percentage ofg/dþ T cells was markedly increased whenin vitrogenerated normal human T-cell blasts were cultured withL. donovaniamastigotes. About 30% of theg/dþ Tcells in the parasite exposed T-cell blasts expressed parasite antigens intracellularly without detectableintracellular parasites. Parasite exposed T-cell blasts had a reduced surface expression of HLA-DR and werelysed by the sorted CD56þ cells. In contrast, neitherL. donovaniamastigotes nor T-cell blasts exposed to heatkilled amastigotes and/or were sensitive to the NK cell-mediated lysis. Of interest is that about 10% CD3þ

peripheral blood T cells in two out of three Indian Kala-azar patients tested expressed intracellularL. donovaniantigens.

Dr Santu Bandyopadhyay, Indian Institute of Chemical Biology, 4, Raja S.C. Mullick Road, Jadavpur,Calcutta 700 032, India

INTRODUCTION

g/dþ T cells have been increasingly detected during variousinfections with pathogens such as mycobacteria [1],Listeriamonocytogenes[2], Trypanosoma cruzi[3], Epstein-Barr virus[4] including Leishmania [5–8]. Several lines of evidencesuggest a first line of defense [2] and a protective role [9, 10]for g/dþ T cells. T cells bearingg/d receptors can discriminateearly between intracellular and extracellular pathogens anddifferentially produce interferon (IFN)-g or interleukin (IL)-4[11]. g/dþ T cells share several features with innate immune cells[12]. With a few exceptions, the antigen recognition byg/dþ Tcells is not MHC-restricted, and the antigen presentation tog/dþ

T cells bypasses the protein processing pathways required for thepeptide association to MHC class I and class II molecules [13]. Inaddition to intact proteins and peptides, soluble nonpeptide anti-gens isolated from mycobacterium have been shown to stimulate amajor population of peripheral bloodg/dþ T cells [14, 15]. Theselow molecular weight nonproteinacious ligands were characterizedas phosphorylated nucleotides [16] as well as several compoundsrelated to prenyl pyrophosphate [17, 18] and were found to be themajor stimulatory components of many pathogenic and nonpatho-genic bacteria and protozoa forg/dþ T cells [5].

The functional significance of target lysis as an NK cell-mediated protective activity in protozoan infection is unknown.However, a role of NK cells in the host recovery from visceralleishmaniasis has been suggested [19, 20].

In the present paper, we demonstrate thatLeishmania dono-vani amastigotes induce the expansion of human peripheralblood g/dþ T cells without infecting them and make themsusceptible to NK cell-mediated lysis.

MATERIALS AND METHODS

Media, reagents and antibodies. Medium RPMI-1640, Medium-199,Freund’s complete and incomplete adjuvants were obtained from GibcoLaboratories (Grand Island, NY, USA). FBS, PHA, PE-conjugatedantirabbit immunoglobulin (Ig) and FITC-conjugated antimouse Igwere obtained from Sigma (St. Louis, MO, USA). Recombinant humaninterleukin (IL)-2 (rhIL-2, specific activity of 2.5(106 U/mg) was purchasedfrom Genzyme Corporation (Cambridge, MA, USA). FITC-conjugatedanti-CD3, anti-CD25, anti-CD19, PE-conjugated anti-CD56, anti-CD33,anti-HLA-DR, purified anti-TCR-g/d1 and purified anti-TCR-a/b1 wereobtained from Becton Dickinson (San Jose, CA, USA). FACSTM

permeabilizing solution was also purchased from Becton Dickinson forpermeabilizing cells for intracellular staining. The hybridoma W6/32which produces a monoclonal antibody that reacts with monomorphic

Scand. J. Immunol.50, 588–595, 1999

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determinants of the HLA-A,B,C molecules [21] was obtained from theAmerican Type Culture Collection (Manassas, VA, USA).

Parasite. TheLeishmania donovani(L. donovani) virulent strain AG83was originally obtained from an Indian Kala-azar patient [22] and main-tained in golden hamsters. Amastigotes were purified from the spleen ofinfected hamsters as described using percoll density gradient centrifugation[23]. Promastigotes were obtained by transforming amastigotes and main-tainedin vitro in Medium-199 supplemented with 8% FBS.

Generation of polyclonal antibodies against L. donovani amastigotes.The polyclonal antibody againstL. donovaniamastigotes was raisedusing a standard protocol. Briefly, purified amastigotes ofL. donovaniwere washed (×2) and resuspended in PBS at a concentration of 2×108

cells/ml. After thorough mixing with Freund’s complete adjuvant (1 : 1),suspensions were injected to healthy rabbits intradermally. After receiv-ing five booster injections (in Freund’s incomplete adjuvant), immunesera were collected and kept at¹708C until used. The antiamastigoteantiserum at a dilution of 1 : 500 gave positive staining with freshlypurified amastigotes ofL. donovaniand was used to detectL. donovaniantigens by flow cytometry. Normal rabbit serum (1 : 500 dilution) wasused as the control for antiamastigote antiserum.

In vitro interaction of L. donovani with T lymphocytes. In vitrogenerated human T-cell blasts were allowed to interact with purifiedL. donovani amastigotes as indicated below. The peripheral bloodmononuclear cells (PBMC) were isolated from heparinized blood ofnormal healthy donors by Ficoll/Hypaque density gradient centrifuga-tion. T-cell blasts were prepared by culturing normal human PBMC withPHA (5(g/ml) for three days and then in the presence of rhIL-2 (50 U/ml)for one more day. T-cell blasts were incubated with purifiedL. donovaniamastigotes at a multiplicity of infection of 10 for 1 day at 378C. Freeamastigotes were removed by low-speed centrifugation, the cells wereresuspended in RPMI-1640 containing 10% FBS, 50 U/ml of rhIL-2 andwere incubated further at 378C, 5% CO2. At different time intervals,cells were withdrawn from culture for Giemsa staining, flow cytometryand cytotoxicity assays. As the appropriate control for purified amasti-gotes, ‘mock’ amastigotes were prepared from noninfected spleens in thesame way and were incubated with T-cell blasts.

Cytotoxicity assays. 51Cr-release assays were performed usingL.donovani amastigotes,L. donovani promastigotes, T-cells blasts,L.donovaniamastigote-exposed T-cell blasts, ‘mock’ amastigote-exposedT-cell blasts, heat killedL. donovaniamastigote-exposed T-cell blastsand K562 cells as targets and the nonadherent peripheral blood mono-nuclear cells (NPBMC) and the sorted CD56þ cells (from NPBMC) aseffectors as described earlier [24, 25]. Briefly, radiolabeled (withNa2

51CrO4) targets (5×105 for promastigotes and amastigotes; 1×104for other targets) were cultured (in a total volume of 1.0 ml of RPMI-1640 containing 10% FBS for promastigotes and amastigotes; in avolume of 0.2 ml of RPMI-1640 containing 10% FBS for other targets)in the presence of various concentrations of effector cells for 18 h at378C in 5% CO2. Cytotoxicity assays were done in triplicate. Afterincubation, cultures were centrifuged, cell-free supernatants (0.5 mlaliquots for promastigotes and amastigotes; 0.1 ml aliquots for othertargets) were collected and their radioactivity was counted in a gammacounter. The percent51Cr–release was calculated by the formula: Thepercent51Cr ¹ release¼ 100 × [(cpm experimental¹ cpm sponta-neous)/(cpm total¹ cpm spontaneous)], where the spontaneous releasewas obtained from target cells incubated with medium alone and the totalrelease was obtained from target cells incubated with 1N HCl. In 18 hassays, the spontaneous release never exceeded 35%.

Flow Cytometry. To determine the specificity of antiamastigoteantiserum freshly purified amastigotes, normal human T-cell blasts

and normal hamaster splenocytes were reacted with the normal rabbitserum (1 : 500 dilution) or antiamastigote antiserum (1 : 500) for 20 min,washed (×2), and stained with the PE-labeled antirabbit Ig (1 : 200dilution) for 15 min. Staining for surface expression of the TCR-g/dand TCR-g/d was performed on T-cell blasts after culturing in thepresence or absence of live or heat killedL. donovaniamastigotes and‘mock’ amastigotes for 2d. Isotype matched control antibodies wereincluded in the staining experiments as negative controls. MoAbs werevisualized by FITC-labeled antimouse Ig. Surface staining was alsoperformed for the expression of MHC class I (W6/32, visualized byFITC-goat antimouse Ig) and class II (anti-HLA-DR-PE) molecules.Parasite-exposed or unexposed T-cell blasts were surface stained for theexpression of CD3, CD25 or TCR-g/d permeabilized (by treatment withFACSTM following manufacturer’s instruction) and then stained forparasite antigens (antiamastigote antiserum followed by treatment withPE-labeled antirabbit Ig). Nonadherent peripheral blood mononuclearcells (NPBMC) of three Visceral leishmaniasis (VL; Kala-azar) patientswere tested for the detection of intracellular parasite antigens. Peripheralblood was collected from VL patients living in Patna, India. Thediagnosis of VL was made after a careful clinical examination andwas confirmed by (a) ELISA with patients’ sera using immobilized crudeantigen fromL. donovanistrain AG83 to capture antibody [26] and (b)by the demonstration of amastigotes in splenic or bone marrow aspirates[27]. These patients had an active disease and their blood was collectedbefore they received any treatment. NPBMC of five normal donors wereused as controls. Cells were surface stained for the expression of CD3 orCD19, permeabilized, then stained for parasite antigens. FITC-or-PE-labeled secondary antibodies were adsorbed over amastigotes to reducenonspecific staining. Single and two colour flow cytometry wereperformed using FACS Calibur (Becton Dickinson) and data wereanalyzed using Cell Quest program.

RESULTS

L. donovaniamastigotes induce expansion ofg/dþ T cellsin vitro

We recently reported that theL. donovaniamastigotes infect thehuman T-cell linein vitro [28]. To extend this study in primaryhuman T lymphocytes,in vitro generated human peripheralblood T-cell blasts were cultured with purifiedL. donovaniamastigotes (live and heat killed) in the presence of rhIL-2. Atdifferent time points, cells were tested for the presence ofintracellular parasites and also surface stained for the expressionof TCR-a/b and -g/d. These experiments were repeated fivetimes with T-cell blasts prepared from five independent donors.One representative experiment is shown in Fig. 1. The back-ground staining, most likely because of nonspecific stickness, ofT-cell blasts cultured with the amastigotes was always slightlyhigher than that of T-cell blasts cultured in media. This non-specific staining could not be completely reduced even by theadsorption of a secondary antibody (FITC-labeled antimouse Ig)over amastigotes. There is always a 2.5–3.6 fold increase ofg/dT cells (depending on the donor) when the T-cell blasts werecultured with the amastigotes although intracellular parasiteswere undetectable. The percentage ofg/d T cells remainedvirtually unaltered when T-cell blasts were cultured with heatkilled L. donovaniamastigotes. The difference inab TCR stainingof T-cell blasts cultured in the presence and absence of amastigotes

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may not be significant. Only a 1.1 fold decrease in theab TCRstaining was detected as opposed to a 3.6 fold increase in theg/dTCR staining. To rule out the possibility that the human TCR-g/dspecific MoAb cross reacts with the parasite resulting in anapparent increase ofg/dþ cells, theL. donovaniparasites werealso stained with the human TCR-g/d specific MoAb. No appreci-able staining was detected (Fig. 1I, J). As the appropriate controlfor purified amastigotes, ‘mock’ amastigotes were prepared fromthe spleens of uninfected hamsters in the same way and wereincubated with T-cell blasts. ‘Mock’ amastigotes did not inducephenotypic alterations in T-cell blasts (not shown).

Coculture withL. donovaniamastigotes reduce the surfaceexpression of HLA-DR on T-cell blasts

After an incubation with liveL. donovaniamastigotes T-cellblasts were monitored for the surface expression of HLA-DR and

MHC class I molecules. Data are presented as two colour dotplots. These experiments were repeated three times and eachtime parasite-exposed T-cell blasts had reduced the surfaceexpression of the HLA-DR compared to unexposed blasts andno significant change was noticed for the MHC class I. Onerepresentative experiment is shown in Fig. 2.

In vitro generated human T-cell blasts expressL. donovaniantigens intracellularly after incubation withL. donovaniamastigotes

To determine the specificity of the antiamastigote antiserum,purified amastigotes, normal human T-cell blasts and normalhamster splenocytes were stained with this antiserum. As shownin Fig 3, 88.4% of the amastigotes was stained strongly with thisantiserum. Normal T-cell blasts had virtually no reactivity, and

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Fig. 1. g/dþ anda/bþT cell responseinduced byL. donovaniamastigotes. T-cellblasts, generated from normal humanPBMC, were incubated for 2 days withmedia alone (A, D, G), liveL. donovaniamastigotes (B, E, H) or heat killedL.donovaniamastigotes (C, F). Cells werethen surface stained for the expression ofTCR g/d or a/b. Staining ofL. donovaniparasites with control MoAb and anti-TCR-g/d-1 are also shown (I, J).

only 3.2% cells of normal hamster splenocytes gave a positivestaining. These data suggest that the antiamastigote antiserumdetects leishmanial antigens efficiently but has no cross reactiv-ity with human cells and that the purified amastigotes contain aminimal contamination of host (hamster) cell proteins resultingin marginal reactivity of the antiserum with normal hamstersplenocytes.

To investigate whether parasite exposed T cells expressL. donovaniantigens, NPBMC of normal donors were incubatedwith L. donovaniamastigotes. Cells were surface stained for theexpression of CD3 and then staining for parasite antigens wereperformed before and after permeabilization of the cells. Asshown in Fig. 4, only a few CD3þ T cells (1.7%) had detectableL. donovanianatigens on their surface. In contrast, 8.4% of theCD3þ T cells were positive forL. donovaniantigens after thepermeabilization. Staining with a normal rabbit serum (negativecontrol for parasite antigen) before or after permeabilization ofNPBMC yielded<2% positive cells.

Two colour flow cytometry was then performed with T-cellblasts incubated with media orL. donovani amastigotes toidentify the cells expressingL. donovani antigens. Surfacestaining was performed for the expression of CD3, CD25 orTCR-g/d and the intracellular staining was performed withthe antiamastigote antiserum for the presence ofL. donovani

antigens. As shown in Fig 5, 16.0% of the CD3þ cells and 9.9%of the CD25þ cells expressedL. donovaniantigens intracellu-larly after incubation withL. donovaniamastigotes. Less than0.3% parasite antigens-positive cells were detected after stainingT-cell blasts (cultured in the presence of amastigotes) with anormal rabbit serum (Fig. 5E). Surface staining for TCR-g/drevealed that T-cell blasts contained between 3 and 6% cellsbearing TCR-g/d. Results of a representative donor is shown inFig. 5D. The percentage of TCR-g/d bearing cells increasedsignificantly (from 6.2% to 19.1%) after incubating the T-cellblasts withL. donovaniamastigotes and 34% of these TCR-g/dpositive cells containedL. donovani antigens intracellularly(Fig. 5H). After incubating T-cell blasts withL. donovaniamastigotes, the TCR-g/d bearing cells were sorted [86.5% ofthe sorted cells were (g/dþ T cells (Fig. 5I)], and Giemsastained for the detection of intracellularL. donovaniamastigotes.Intracellular amastigotes were undetectable in the sortedg/dþ

T cells.

CD3þ peripheral blood T cells of Indian Kala-azar patientsexpressL. donovaniantigens intracellularly

NPBMC were isolated from three Indian Kala-azar (KA) patientsand Giemsa stained for the presence of intracellular amastigotes.NPBMC of KA patients were also surface stained for theexpression of CD3 or CD19, permeabilized and stained forintracellular L. donovani antigens. For comparison, NPBMCfrom normal donors were also isolated and stained in the similarmanner. NPBMC of none of the KA patients (n¼ 3) or normaldonors (n¼ 5) tested had intracellular amastigotes. However, anappreciable proportion of CD3þ peripheral blood T cells (,10%)of two KA patients out of three tested had intracellularL. donovaniantigens. None of these KA patients’ CD19þ cells had detectableintracellularL. donovaniantigens. Dot plots of one representativeKA patient’s NPBMC is shown in Fig. 6(A, B). CD3þ T cells orCD19þ B cells of all three normal donors also tested negative forintracellularL. donovaniantigens. Dot plots of one representativenormal donor’s NPBMC is shown in Fig. 6(C, D).

Participation of CD56þ cells in lysis of T cell blasts exposed toL. donovani

T-cell blasts exposed toL. donovaniamastigotes were suscep-tible to NPBMC-mediated lysisin vitro. To investigate whetherNK cells were responsible for this killing, CD56þ cells weresorted from NPBMC of normal donors and were used aseffectors. Sorted population always contained>99 % CD56þ

cells (not shown). Appreciable lysis ofL. donovani-exposedT-cell blasts was detectable by NPBMC as well as by the sortedCD56þ cells (Table 1). However NPBMC or sorted CD56þ cellswere unable to lyseL. donovanipromastigotes,L. donovaniamastigotes, T-cell blasts incubated with heat killedL. donovaniamastigotes, T-cell blasts incubated with media, or T-cell blastsincubated with ‘mock’ amastigotes (Table 1). NK-susceptible



Fig. 2. Effects ofL. donovaniamastigotes on the surface expression ofMHC class I and HLA-DR antigens by T-cell blasts.In vitrogenerated T-cell blasts were incubated with media alone (C, D) orwith L. donovaniamastigotes (A, B) for 2 days. Cells were surfacestained for the expression of MHC class I and HLA-DR antigens. Thepercentage of cells expressing both antigens is given on the upperright panels. Note that the intensity of MHC class I on T-cell blastsincubated with media orL. donovaniamastigotes remainedunaltered. In contrast, T-cell blasts incubated withL. donovaniamastigotes had reduced expression of HLA-DR (mean fluorescenceintensity, 195) compared to T-cell blasts incubated with media alone(mean fluorescence intensity, 337).

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Fig. 3. Determination of specificity ofantiamastigote antiserum. PurifiedL. donovaniamastigotes (A, B), normalhuman T cell blasts (C, D) and normalhamster splenocytes (E, F) were stained withnormal rabbit serum (A, C, E) or with anti-L. donovaniamastigote antiserum (B, D, F).The percentage of positive cells is given onthe lower right panels.

Fig. 4. CD3þ resting T cells expressL. donovaniantigens intracellularly afterinvitro incubation withL. donovaniamastigotes. Nonadherent peripheral bloodmononuclear cells (NPBMC) of normaldonors were incubated withL. donovaniamastigotes for 2d, and free parasites wereremoved by low-speed centrifugation. Aftersurface staining for CD3, cells were stainedwith normal rabbit serum (A, C) andantiamastigote antiserum (B, D) before(A, B) and after permeabilization (C, D).The percentage of cells expressing only CD3antigen is shown on lower right panels, andthe percentage of cells expressing both CD3and parasite antigens is shown on upperright panels.

K562 target cells were used as a positive control and, asexpected, sorted CD56þ cells induced high degree of lysis ofK562 (59.9%) at an E/T ratio of 2 : 1 compared to 77.6% lysis byNPBMC at an E/T ratio of 30 : 1 (Table 1).

DISCUSSION

We previously reported that human T-cell line HUT-78 could beinfectedin vitro by L. donovaniamastigotes [28]. It is thereforeof interest to study the interaction of this parasite with primaryhuman T cells.In vitro generated T-cell blasts from normalhuman PBMC were used as a source of T cells. A significantelevation in the percentage ofg/dþ T cells was found uponcoculture of T cell blasts withL. donovaniamastigotes. Althoughintracellular parasites were undetectable in parasite-exposedT-cell blasts, about 30% of theg/dþ T cells expressed intra-cellular L. donovaniantigens.Leishmania-exposed T-cell blastshad reduced surface expression of HLA-DR and were susceptibleto NK cell-mediated lysis. Of interest, heat killedL. donovaniamastigotes or ‘mock’ amastigotes prepared from spleens of



Fig. 5. IntracellularL. donovaniantigensexpression by T cell blasts after incubationwith L. donovaniamastigotes. T cell blastswere incubated with media (A, B, C, D) orL. donovaniamastigotes (E, F, G, H, I) for2d. After surface staining for the expressionof CD3 (B, F), CD25 (C, G) org/d TCR(D, H), cells were permeabilized and stainedwith control rabbit serum (A,E) or withantiamastigote antiserum. T cells bearingg/d TCR were sorted from T-cell blasts afterincubation withL. donovaniamastigotes (I)to examine the presence of intracellularamastigotes.

Fig. 6. IntracellularL. donovaniantigens expression by CD3þ restingT cells of a Kala-azar patient. NPBMC from a Kala-azar patient(A, B) and from a normal donor (C, D) were surface stained for theexpression of CD3 (A, C) or CD19 (B, D), permeabilized and thenstained with antiamastigote antiserum. The patient was confirmed tohave visceral leishmaniasis by the demonstration of amastigotes insplenic aspirates.

uninfected hamsters induced neither the expansion ofg/dþ T cellsnor the sensitivity to NK cell-mediated lysis.

The expansion ofg/dþ T cells has been reported in human andmurine leishmaniasisin vivo [6–8]. Although the functionalsignificance of target lysis as an NK cell-mediated protectiveactivity in protozoan infection is unknown, a role for the NKcells in the host recovery from visceral leishmaniasis, particu-larly in control of infection in the spleen, in the mouse model wassuggested [19]. The rapid elevation and diminution of thecytotoxic response induced byLeishmaniaalso suggested theparticipation of NK cells in the innate phase of host resistanceagainst protozoa [20]. In many cases, the production of IFN-g

and the macrophage microbicidal activity induced by IFN-g, notthe target lysis is probably the primary resistance mechanism ofNK cells in protozoan infection. Although it had been difficult todemonstrateLeishmaniainduced NK cell activation using theSCID model [29, 30], the existence of such a T cell-independentmechanism was recently suggested by Laskay and colleagues[31] who found that the parasite numbers were increased inL. major-infected SCID mice after treatment with antisera toeither NK cells (asialo GM1) or IFN-g. On the other hand, NKcells appeared to play a relatively weak role in the resolution ofexperimentalLeishmaniainfection in the liver [32, 33]. Also, NKcells did not exert detectable antileishmanial activity in nudemice [32].

Destruction of protozoa by NK cells could occur either by thelysis of extracellular organisms or by the destruction of infectedcells. NK cells from mice infected withTrypanosoma cruzi,thecausative agent of Chagas disease, exhibited significant lyticactivity against extracellular trypomastigotes and epimastigotes[34]. The NK cell-mediated parasite destruction was also docu-mented with extracellular tachyzoites ofToxoplasma gondii[35]or extracellular Tetrahymena pyriformis[36]. Macrophagesinfected with, T. gondii [37] or Leishmania major[38] were

also susceptible to a NK cell-mediated lysis. In most instances,such activity has been demonstrated only after activation ofeffector cells with lymphokines. Our data demonstrate thatprimary human T cells incubated with this parasite are suscep-tible to NK cell-mediated lysis without prior activation withlymphokines. The data is of interest as it could represent amechanism for immunosuppression observed in visceral leish-maniasis. However, the role of endogenously produced cyto-kines, in particular IFN-a in this NK cell-mediated cytotoxicitycan not be ruled out. We previously reported that NK cellsselectively lyse virus-infected cells via a mechanism that is atleast in part dependent on the production of IFN-a a potentstimulator of NK cell activity [24]. Interestingly, IFN-a wasdetected in the NK assay supernatants where T cell-blastsexposed toL. donovaniwere used as targets and NPBMC aseffector cells (not shown).

In conclusion, our data indicate thatL. donovaniamastigotesinteract with a significant proportion ofg/dþ human T cells andmake them susceptible to NK cell mediated lysis.

ACKNOWLEDGMENTS

The work was supported by the Council of Scientific & IndustrialResearch, Department of Science & Technology and the Depart-ment of Biotechnology, the Government of India. We thank S.K.Sahoo and D. Das for art work, and A. Manna for typing themanuscript.

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Table 1. T cell blasts incubated withL. donovaniamastigotes are sensitive to CD56þ NK cell-mediated lysis*

Percent51Cr-release from

T cell blastsT cell blasts incubated T cellincubated with blasts

T cell blasts with heat killed incubatedEffector Incubated L. donovani L. donovani with ‘mock’ L. donovani L.donovaniCells E/T ratio with media amastigotes amastigotes amastigotes K562 amastigotes promastigotes

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Leishmania donovani Parasites Interact with γ/δ+ Human Peripheral Blood T Cells and Induce Susceptibility to NK Cell-Mediated Lysis