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Neurotrophic receptors TrkA and TrkC double up as receptors that Trypanosoma cruzi uses to invade cells and as autoantigen in T. cruzi-infected individuals (with Chagas’ disease). Consequently, autoantibodies against TrkA and TrkC (ATA) potently block T. cruzi invasion in vitro and in ATA-immunized mice. Thus, ATA could keep T. cruzi invasion in check in Chagas’ disease. However, ATA has been examined only in patients with chronic Chagas’ disease. To determine whether ATA potentially participate in the early stage of infection, we analysed the sera of 15 patients with acute Chagas’ disease, 4–66 years of age. We find that all sera contain high antibody titres to TrkA, TrkB and TrkC, but not to other growth factor receptors, indicating that ATA are produced relatively soon after T. cruzi infection by an age-independent process. One individual, who acquired the disease after an accidental laboratory infection, converted to Trk-antibody (Ab)-seronegative when progressing to the chronic phase. ATA from acute patients were of low avidity (K0 <24.8 × 10−8 m) and of IgM and IgA isotypes. In contrast, ATA from chronic patients were of high avidity (Ko = 1.4 to 4.5 × 10−8 m) and of the IgG2 isotype. Therefore, ATA underwent affinity maturation and class switch when patients progressed from acute to chronic disease. Thus, it may be that Trk autoimmunity, which starts in the acute Chagas’ disease, plays a role in attenuating parasitemia and tissue parasitism that characterizes the acute/ chronic phase transition of Chagas’ disease.
Acute Chagas’ disease commonly starts when T. cruzi metacyclic trypomastigotes, released in the faeces and urine of reduviid bugs taking a blood meal, invade keratinocytes and other cell types in the skin and mucosa [1–3]. Inside the host cells, trypomastigotes differentiate into amastigotes and undergo several cycles of replication by binary fission before redifferentiation into the non-dividing trypomastigotes. Upon exiting infected cells, trypomastigotes migrate through the extracellular matrix to invade neighbouring cells or, through the circulation, distant cells in the heart, gastrointestinal tract, central nervous system and other organs. Repeated cellular cycles of T. cruzi invasion through the body are a characteristic feature of acute Chagas’ disease, which lasts only a few months.
Acute disease ends when parasitemia becomes undetectable by optical microscopy, setting the stage for the onset of the chronic phase of infection. This can be subdivided in two clinical forms: 1) indeterminate, when patients are asymptomatic and exhibit normal heart and digestive tract functions evaluated by electrocardiogram and radiography. And 2) symptomatic, when patients, for reasons that remain unknown, present pathological alterations that lead to electrical disturbances and enlargement of the heart (cardiomegaly), oesophagus (megaoesophagus) and/ or colon (megacolon), accompanied by strong inflammation, fibrosis and destruction of the peripheral nervous system [4, 5]. Chronic Chagas’ infection, including those individuals in the indeterminate form, may last many years or decades.
Innate and adaptive immunity play a critical role in reducing parasite growth in the acute/ chronic phase transition of Chagas’ disease and in maintaining low parasite burden that characterizes chronically infected individuals . However, the relevant antigens, specific antigenic determinants and corresponding immune response governing these mechanisms remain incompletely understood.
Recently, we discovered that sera of ~80% patients with chronic Chagas’ disease contain autoantibodies (ATA) to TrkA, TrkB and TrkC, the tyrosine kinase receptors of the neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), respectively , that underlie development and repair of the nervous system [8, 9]. As T. cruzi uses TrkA and TrkC to enter and activate neurons and glial cells [10–12], binding of ATA to TrkA and TrkC blocks invasion of neuronal, glial and non-neural cells in culture by the parasite . Furthermore, when passively administered to mice, ATA potently blocked parasitemia, pathology and mortality . Thus, ATA may represent a mechanism responsible for the low tissue parasitism that distinguishes chronic Chagas’ disease.
If ATA reduces cellular invasion, underlying low tissue parasitism, then Trk autoimmunity should emerge in the acute phase of Chagas’ disease, as it ends with a drastic decline in parasitemia and tissue parasite load. We confirm this prediction by showing here that ATA is generated in the acute phase of Chagas’ disease and that they remain in most chronically infected individuals, supporting the concept that Trk autoimmunity may be beneficial.
The sera from patients with acute Chagas’ disease, all from the states of Minas Gerais, Bahia, and Goiás, Brazil, were described in a previous study  except for serum samples collected during 1.9 month, 7.9 months and 15.15 years from an individual accidentally infected with T. cruzi, which were kindly made available to us for this study. In all patients, T. cruzi was detected by microscopic examination of blood. The sera from chronic indeterminate disease and non-chagasic sera were also from previous studies . Prior to use, the sera, stored in 50% glycerol at 4 °C, were centrifuged at 1,200 g for 10 min and diluted in appropriate buffers, as described later. Ethical approval was obtained from the Human Investigation Review Committee of Tufts Medical Center.
Microtitre wells were coated overnight at 4 °C with recombinant extracellular domain (ECD) of human TrkA, TrkB and TrkC receptors fused to the Fc region of human IgG (400 ng/ ml) (R&D Systems, Minneapolis, MN, USA) as described earlier , blocked with 5% goat serum (2 h, 37 °C), followed by chagasic sera diluted at 1:200 (unless otherwise indicated) in 5% bovine serum albumin/ phosphate-buffered saline pH 7.2 containing 0.1% Tween-20, washed and developed with alkaline phosphatase (AP)-labelled secondary relevant antibody. To determine the antibody titres against T. cruzi, trypomastigotes were obtained from cellular cultures, lysed by repeated cycles of freeze/ thaw, cleared of debris by centrifugation (12,000 g, 10 min), layered on microtitre wells (500 ng / ml, 4 °C) and probed with chagasic sera, as described earlier. ATA isotyping was performed by ELISA with commercially available kits (Sigma-Aldrich, St Louis, MO, USA) based on mouse mAb to human IgG isotypes and goat antibodies specific to IgA and IgM.
This was determined as previously described [11, 15] except that we used ELISA instead of ligand blotting to obtain avidity measurements. In brief, microtitre wells were coated overnight with Fc chimera of Trk receptors-ECD (TrkA, TrkB, and TrkC) or control receptor (p75NTR), blocked with 5% goat serum (2 h, 37 °C), washed and incubated with sera (1:200, 2 h) without pre-incubation or after pre-incubation (4 °C, overnight) with various concentrations of soluble receptor-ECD and developed with AP-labelled secondary relevant antibody, as described earlier. Avidity to TrkA was also determined in an affinity-purified rabbit TrkA antibody (Abcam, Cambridge, MA, USA). Avidity measurements and plots were obtained with the prism 4.0 program (GraphPad Software, La Jolla, CA, USA).
ELISA assays revealed that the sera of the 15 individuals with acute Chagas’ disease analysed in this study contained antibodies to the ECD of TrkAECD, TrkBECD and TrkCECD, with mean titres of 2.22 ± 0.1, 1.95 ± 0.07 and 2.07 ± 0.1, respectively, compared to 0.12 ± 0.05, 0.06 ± 0.01 and 0.07 ± 0.1 for the 30 sera from non-chagasic individuals (Fig. 1A). Antibody titres against the extracellular domain of four other neurotrophic factors (transforming growth factor-β receptor II, TGFβR-II; pan-neurotrophin receptor p75, p75NTR; glial cell-derived neurotrophic receptorα-1, GFRα-1; and tyrosine kinase receptor rearranged in transformation (RET) of glial cell-line derived neurotrophic factor family ligands, rearranged in transformation (RET) of were within the range of non-chagasic sera titres (Fig. 1A).
The mean titres of antibodies against TrkA, TrkB and TrkC in all acute chagasic sera were three standard deviations above the mean titres of non-chagasic sera and thus were considered Trk-Ab-seropositive (Fig. 1A,B). This was in contrast to the sera of chronic chagasic individuals in the indeterminate phase, in which case 6 out of 26 (20%) sera were considered Trk-Ab-seronegative (Fig. 1A,B), thereby confirming previous results . Notably, sera from patients with acute and chronic Chagas’ disease seropositive for TrkAECD were also seropositive for TrkBECD and TrkCECD, while the sera from chronic patients seronegative for TrkAECD were also seronegative for the other two Trk receptors (Fig. 1A–C). This suggests that the TrkA epitope(s) recognized by the autoantibodies is (are) similar to the one(s) in TrkB and TrkC. Also of interest is the finding that the mean antibody titres to TrkA and TrkB in the sera of acute patients were statistically significantly higher than the corresponding titres in Trk-seropositive chronic chagasic individuals (Fig. 1D).
Autoantibodies to TrkA, TrkB and TrkC were present in patients with acute Chagas’ disease analysed here ranging in age from 4 to 66 (Fig. 2A), with an average of 20.8 ± 17.1 years (Fig. 2D). This is in contrast to patients with Trk-Ab-seropositive chronic Chagas’ disease, who were older (23 to 60 years of age, average of 40.5 ± 12.4 years) but similar to the average age of patients with Trk-Ab-seronegative chronic Chagas’ disease (43.2 ± 7.9 years) (Fig. 2A–D). Thus, ATA in patients with acute Chagas’ disease emerge by an age-independent process.
Trk autoantibodies from patients with acute disease were of the IgA and IgM isotype (Fig. 3A, sera from nine patients) and of low avidity (<24.8 × 10−8 m, sera from three patients), (Fig. 3A,C) and (Table 1), contrary to the autoantibodies from patients with chronic Chagas’ disease, which were exclusively IgG2  and of relatively high avidity (1.4 to 4.5 × 10−8 m) (Fig. 3C,D). The avidity of ATA from patients with chronic Chagas’ disease was similar to that of a commercial rabbit antibody to TrkA (Fig. 3E). Thus, ATA must undergo antibody class switch from IgA and IgM IgG and affinity maturation (many-fold increase) when patients progress from acute to chronic disease.
ATA was detected less than two months after an individual acquired Chagas’ disease by an accidental laboratory infection with T. cruzi, an event that is not uncommon [1, 3] (Fig. 4A). Antibody titre to the three Trk receptors decreased sharply six months afterwards, when the patient was shifting to the chronic phase, and became undetectable 15–16 years after the start of the accidental infection (Fig. 4A), while the patient remains asymptomatic. The patient continues to be infected with T. cruzi and thus with Chagas’ infection because of the high antibody titres to the parasite >15 years after the onset of infection (Fig. 4B). This illustrates that a Trk-Ab-seropositive patient in the acute phase can be converted to Trk-Ab-seronegative, consistent with 100% patients bearing acute Chagas’ disease Trk autoantibodies and with some patients (~20%) converting to Trk-Ab-seronegative when progressing to the chronic phase of the disease.
In sum, our results show that individuals acutely infected with T. cruzi produce autoantibodies specific to TrkA, TrkB and TrkC, the tyrosine kinase receptors of the neurotrophins NGF, BDNF and NT-3 that regulate development and repair of central and peripheral nervous system [9, 16]. They were elicited by patients as young as a 4-year-old child and as aged as a 66 year-old adult and, thus, by an age-independent process. Given that acute infection starts after the parasite gains access to humans and lasts only a few months, the Trk autoantibodies should arise relatively soon after T. cruzi infection. This was dramatically demonstrated in a patient with Chagas’ disease accidentally infected in a laboratory, as high titres of antibodies were evident less than two months after the accident (Fig. 4A). The Trk autoantibodies from patients with acute Chagas’ disease bear characteristics of antibodies produced in acute disease or after primary immunization (IgM and IgA isotype and low avidity).
Unravelling the immunochemistry and biology of these neurotrophin receptor autoantibodies is of great interest because one of the autoantigens – TrkA – serves as a vehicle for T. cruzi, via its trans-sialidase/ neurotrophic factor, to promote neuron survival  and to invade cells , while another autoantigen – TrkC – is used to induce survival and differentiation of neurons and Schwann cells . ATA isolated from sera of patients in the indeterminate phase of Chagas’ disease compete with T. cruzi for Trk binding and inhibit infection in vitro and in vivo . Consequently, ATA could modulate Chagas’ disease progression by reducing tissue parasitism in chronically infected individuals. In view of the present findings, ATA could play a role in the dramatic decline in tissue parasitism when patients progress from acute to chronic disease.
This work was supported by NIH Grants NS40574 and NS42960.