Disclosing Hemolymph Collection and Inoculation of Metarhizium Blastospores into Rhipicephalus Microplus Ticks Towards Invertebrate Pathology Studies

Summary

Analysis of tick hemolymph represents an important source of information on how some pathogens cause disease and how ticks immunologically respond to this infection. The present study demonstrates how to inoculate fungal propagules and collect hemolymph from Rhipicephalus microplus engorged females.

Abstract

Ticks are obligate hematophagous ectoparasites and Rhipicephalus microplus has great importance in veterinary medicine because it causes anemia, weight loss, depreciation of the animals' leather and also can act as a vector of several pathogens. Due to the exorbitant costs to control these parasites, damage to the environment caused by the inappropriate use of chemical acaricides, and the increased resistance against traditional parasiticides, alternative control of ticks, by the use of entomopathogenic fungi, for example, has been considered an interesting approach. Nevertheless, few studies have demonstrated how the tick's immune system acts to fight these entomopathogens. Therefore, this protocol demonstrates two methods used for entomopathogen inoculation into engorged females and two techniques used for hemolymph collection and hemocytes harvesting. Inoculation of pathogens at the leg insertion in the tick female's body allows evaluation of females biologic parameters unlike the inoculation between the scutum and capitulum, which frequently damages Gené's organ. Dorsal hemolymph collection yielded a higher volume recovery than collection through the legs. Some limitations of tick hemolymph collection and processing include i) high rates of hemocytes' disruption, ii) hemolymph contamination with disrupted midgut, and iii) low hemolymph volume recovery. When hemolymph is collected through the leg cutting, the hemolymph takes time to accumulate at the leg opening, favoring the clotting process. In addition, fewer hemocytes are obtained in the collection through the leg compared to the dorsal collection, even though the first method is considered easier to be performed. Understanding the immune response in ticks mediated by entomopathogenic agents helps to unveil their pathogenesis and develop new targets for tick control. The inoculation processes described here require very low technological resources and can be used not only to expose ticks to pathogenic microorganisms. Similarly, the collection of tick hemolymph may represent the first step for many physiological studies.

Introduction

The cattle tick, Rhipicephalus microplus, is an hematophagus ectoparasite with an enormous negative impact on livestock in tropical areas. This tick is the vector of pathogenic agents such as Babesia bovis, Babesia bigemina, and Anaplasma marginale that, combined with the direct hemofeeding damage, can reduce milk and meat production, cause anemia and ultimately death. Losses caused by this ectoparasite were estimated in 3.24 billion dollars annually in Brazil¹. Sustainable methods are demanded and the use of entomopathogenic agents is considered a promising alternative to reduce the use of chemical acaricides^2,3,4.

Entomopathogenic fungi, such as Metarhizium spp., are natural enemies of arthropods including ticks, and some isolates can be used as biocontrollers. These pathogens actively infect the host through the cuticle and colonize their body^2,5,6. As the infection develops, cellular and humoral responses are mediated by the tick immune system. Analysis of the tick hemolymph is reported as a useful tool to evaluate the immune responses after the challenging with pathogens^7,8.

Arthropods' immune response is divided into humoral and cellular responses. The humoral response involves hemagglutination processes and production of antimicrobial proteins/peptides, whereas the cellular immune response is performed through the hemocytes. These cells are present in the hemolymph from all arthropods and are reported to develop an expressive role in studies involving innate immune response⁹, as it is directly related to the phagocytosis and encapsulation processes. Accordingly, studies about hemocytes can help to investigate death pathway and understand processes such as autophagy, apoptosis, and necrosis. In some invertebrates as bivalves, the hemocytes' collection faces limitations like cell disruption, low hemolymph volume obtained, and low concentration of recovered cells¹⁰. Very frequently, depending on the methodology applied, reduced concentration of cells is obtained, impacting directly on cells quantification and analysis.

The number of hemocytes circulating in the hemolymph is variable among different arthropods and it can change in the same species due to different physiological stages such as sex, age, and the arthropod's developmental stage¹¹. Hemocytes can also be found adhered to some organs and be released into circulation just after the infection process¹¹. Nevertheless, most studies reported use insects, while ticks remain less explored regarding their physiology and pathology. Despite pathogen inoculation and hemolymph collection in ticks are less used techniques, establishing standard methods helps the development of more accurate studies.

The aim of the present study was to compare the most used methods for hemolymph collection and inoculation of pathogens into R. microplus ticks, evaluating the efficacy in the hemolymph acquisition and hemocytes concentration.

Protocol

Ticks used in the present study were obtained from an artificial colony, mantained at Federal Rural University of Rio de Janeiro, which methods have been approved by the Committee on Ethics for the Use of Vertebrate Animals (CEUA-IV/UFRRJ #037/2014).

1. Tick engorged females

1. After tick gathering, wash engorged females using tap water and immerse them in 0.5% (v/v) sodium hypochlorite solution for 3 min in a 500 mL glass beaker recipient, for cuticle hygiene (Figure 1), then dry all females using sterile paper towels (Figure 2).
2. Divide females in homogeneous weighted groups (with 20 females each): one group without any treatment, one control group inoculated with 5 µL of 0.1% polyoxyethylene sorbitan monooleate aqueous solution (v/v), and one infected-group inoculated with 5 µL at 1.0 x 10⁷ blastospores mL^-1.
   NOTE: Only untreated ticks were used for volume and hemocyte concentration. Three replicates of each group were performed.

2. Pathogens inoculation between the scutum and capitulum

NOTE: In the present study, entomopathogenic fungal suspensions were used as an example.

1. Suspend fungal blastospores in 1 mL of 0.1% polyoxyethylene sorbitan monooleate aqueous solution (v/v) and adjust to a final concentration of 1.0 x 10⁷ blastospores mL^-1. Add an aliquot of 5 µL Metarhizium blastospores (Figure 3) suspension to the surface of a plastic paraffin film.
   NOTE: Metarhizium blastospores suspension was adjusted using a Neubauer´s chamber. To speed the inoculation process, multiple bubbles can be added to the plastic paraffin film surface, each bubble corresponding to 5 µL.
2. Use a 1 mL ultra-fine insulin syringe and a 0.3 mm needle to pull the suspension and inoculate it into the tick. Remember to take all the air out of the syringe before using it.
3. Inoculate 5 µL of fungal suspension into the tick female between the scutum and capitulum. Inoculate females from the control group with 5 µL of 0.1% polyoxyethylene sorbitan monooleate aqueous solution (v/v) with no fungus.
   NOTE: A small volume of hemolymph may be present on the foramen after the needle insertion. Be careful to not inoculate air.

3. Inoculation of entomopathogenic fungi between the leg thigh and the tick female's body

1. Inoculate females with fungal suspension (5 µL at 1.0 x 10⁷ blastospores mL^-1) between the leg tigh and the female's body, using a 1 mL ultra-fine insulin syringe coupled to a 0.3 mm needle. Inoculate females from the control group with 5 µL of 0.1% polyoxyethylene sorbitan monooleate aqueous solution (v/v).
   NOTE: When Metarhizium blastospores inoculation is performed between the leg tigh and the tick female's body, a small volume of hemolymph can be present on the tigh after the needle insertion. Be careful to not inoculate air.

4. Dorsal hemolymph collection

1. Use the rubber part of a winged infusion set, a 0.3 mm capillary tube, and a filtered tip to perform the hemolymph collection.
2. Disrupt the female dorsal cuticle using a 0.3 mm needle.
3. After disruption, apply gentle pressure on the anterior part of the tick body. Observe an almost transparent liquid pulling out of the disruption site. Collect the hemolymph by sucking the liquid through the filter tip coupled to the rubber part of a winged infusion set, and a 0.3 mm capillary tube.
   NOTE: Do not press the tick's body hardly during its immobilization because this may disrupt the midgut and contaminate the hemolymph. Wait until gentle pressure can expel the fluid without contamination.

5. Hemolymph collection from the tick leg

1. Immobilize the tick, cut a piece of a front leg with a pair of scissors.
   NOTE: One or more legs can be cut, as well as, the same leg can be cut more than one time.
2. Apply gentle pressure on the tick's posterior body part. Observe a transparent liquid bubble that shows up on the cut site and collect it with the capillary tube, as described in step 4.3.
   NOTE: Do not press the tick's body hardly during its immobilization because this may disrupt the midgut and contaminate the hemolymph. Wait until gentle pressure can expel the fluid without contamination.

6. Hemolymph processing

1. After hemolymph collection, deposit it in 1.5 mL microtubes previously filled with 30 µL protease cocktail and 82 µL saline buffer. Keep the microtubes on ice throughout the hemolymph collection.
2. Centrifuge samples (500 x g for 3 min at 4 °C). A soft pellet of hemocytes will be formed after hemolymph centrifugation.
   NOTE: For hemolymph quantification, quantify the hemolymph volume obtained by counting the total volume inside the microtube and discounting the volume of protease cocktail and saline buffer.
3. Carefully remove the supernatant (cell-free hemolymph). Gently resuspend the cells in, for example, Leibovitz's L-15 culture adjusted to pH 7.0-7.2. Quantify hemocytes by placing 10 µL of resuspended hemocytes in a Neubauer chamber.

7. Hemocyte sampling slide preparation

1. Cut the tick's front leg with a pair of scissors.
2. Apply gentle pressure on the tick's posterior body part. Observe a transparent liquid bubble that shows up on the cut site.
3. Apply the hemolymph drops directly on clean microscope slides, after that, use appropriated methods to stain the cells.
   1. To stain hemocytes using Giemsa, air-dry the hemolymph on the slide for 30 min, fix it at room temperature with methanol for 3 min, and stain in Giemsa solution (1:9 ratio of Sorensen's buffer solution, pH 7.2) for 30 min at room temperature. Wash the slides with running water to remove the excess of dye, air-dry the slide and evaluate the cells at 400x in an optical microscope.

Results

This article approaches inoculation and hemolymph collection methods applied to ticks. After the inoculation between the leg thigh and the tick female's body, some fluid (hemolymph) can be secreted during the process; however, it is important to note that when the inoculation is finished, no liquid or tissues were present in the needle tip or at the inoculation site, ensuring that the fungal suspension was completely inoculated. When the inoculation process was correctly performed, the ne...

Discussion

Inoculation of pathogens is useful when the study aims to investigate the in vivo action of microorganisms in experimental arthropod models because it assures that the pathogen is inside the host. The technique can also be applied to inoculate molecules such as RNA interference (RNAi). Inoculation between the scutum and capitulum is considered easier to perform but frequently damages Gené's organ, impairing the eggs viability^12,13. Gené's organ ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Alkaline Hypochlorite solution	Sigma-Aldrich	A1727
D-(+)-Glucose	Sigma-Aldrich	G8270-1KG
EDTA	Synth	2706
Fetal Bovine Serum	Gibco	16000036
Flexible rubber	BD
Giemsa stain	Sigma-Aldrich	48900-500ML-F
Glass capillary	CTechGlass	CT95-02
Insulin syringe (needle)	BD	SKU: 324910
KH2PO4	Vetec	60REAVET014512
Leibovitz's L-15 culture medium	Gibco	11415-064
Methanol	Sigma-Aldrich	34860-1L-R
Microscope slides	Kasvi	K5-7105
Microtubes	BRAND	Z336769-1PAK
Na2HPO4	Vetec	60REAVET014593
NaCl	Sigma-Aldrich	S7653-1KG
Neubauer chamber	Kasvi	K5-0111
Penicillin	Gibco	15140163
Protease inhibitor cocktail	Sigma-Aldrich	P2714
Tween 80	Vetec	60REAVET003662