ORIGINAL RESEARCH

Temporal dynamics of cytokines in the blood of rats with experimentally induced autoimmune encephalomyelitis

Pozdniakova NV1, Turobov VI2, Garanina EE3, Ryabaya OA1, Biryukova YuK4, Minkevich NI2, Trubnikova EV5, Shevelev AB6, Kuznetsova TV7, Belyakova AV4, Udovichenko IP2,8
About authors

1 Blokhin National Medical Research Center of Oncology, Moscow

2 Shemyakin–Ovchinnikov Institute of Bioorganic Chemistry of the Russian Academy of Sciences, Pushchino branch, Russia

3 Кazan (Volga Region) Federal University, Kazan, Russia

4 Chumakov Federal Scientific Center for Research and Development of Immune-and-Biological Products of Russian Academy of Sciences, Moscow

5 Kursk State University, Kursk

6 Emanuel Institute of Biochemical Physics of the Russian Academy of Sciences, Moscow, Russia

7 Vavilov Institute of General Genetics, Russian Academy of Sciences, Moscow, Russia

8 Pushchino State Institute of Natural Sciences, Pushchino, Russia

Correspondence should be addressed: Igor Udovichenko
Pr-t Nauki, d. 6, Puschino, Moscow oblast, Russia, 142290; ur.xednay@1oknehcivodui

About paper

Funding: this work was supported by the Ministry of Education and Science of the Russian Federation (Grant agreement 14.607.21.0133 dated October 27, 2015, ID RFMEFI60715X0133).

Acknowledgements: the authors thank Boris Shevelev for help in immunization of the animals.

All authors' contribution to this work is equal: selection and analysis of literature, research planning, data collection, analysis, and interpretation, drafting of a manuscript, editing.

Received: 2017-12-15 Accepted: 2017-12-20 Published online: 2018-01-25
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Multiple sclerosis (MP) is a sever neurodegenerative autoimmune disorder. Due to its high prevalence and the severity of symptoms causing partial or complete loss of mobility, multiple sclerosis remains a pressing problem, prompting a search for new therapies. Most patients with MS completely loose the mobility 25 years after the onset of the disease. More than a half of MS patients become dependent on crutches 15 years after appearance of the first symptoms. To date, there is no effective causal treatment for MS.

Usually the disease strikes at young age: 70 % to 80 % of patients suffer the first symptoms of MS between 20 and 40 years of age [1]. MS is diagnosed by neurological examinations, magnetic resonance imaging of the central nervous system, and by biopsy or autopsy [2]. MS has numerous clinical manifestations indicating damage to the spinal cord, the brain, cranial nerves, the cerebellum, and cognitive function. Current diagnostics are insufficient for accurate estimation of MS severity. MRI, electroencephalography and lumbar puncture can still be inconclusive, in spite of providing valuable information about patient’s condition. In patients with MS, many symptoms can be caused by infection, vascular pathology, or autoimmune comorbidities [3].

There are four types of MS: relapsing-remitting (RRMS, alternating periods of relapses and remissions) occurring in 80 % to 85 % of patients; primary progressive (PPMS) occurring in 10 % to 15 % of patients; progressive-relapsing (PRMS) — in 5 % of patients; and secondary-progressive (SPMS) [4, 5]. About half of patients with RRMS develop symptoms of SPMS 10 years after the onset of the disease. Over 90 % of patients with RPMS eventually demonstrate SPMS symptoms [6].

The hallmark of MS is destruction of the myelin sheaths of neurons in the central nervous system caused by clustering T- and B-cells. Another typical feature of this disease is accumulation of oligoclonal antibodies in the cerebrospinal fluid. It is not clear, though, how and where the clonal expansion of lymphocytes specific for myelin basic protein is initially triggered. We do not know yet whether it happens in the CNS, where the myelin sheath is directly involved, or outside of it, with autoreactive species migrating to the CNS from other places [7].

Development of effective MS treatments is impossible without animal models accurately replicating the course of the disease in humans, such as experimental autoimmune encephalomyelitis (EAE) of rats and mice. EAE is induced by injecting myelin or basic myelin protein (MBP) suspensions in incomplete Freund’s adjuvant into the hind footpads of rodents [8]. One month after immunization the mice develop hind limb paralysis which lasts for 4–6 months [9]. In Dark Agouti (DA) rats, EAE progresses more rapidly (paralysis sets in on days 10–11 and lasts until day 14). The key difference of EAE in animals from MS in humans is full recovery of rodents, which is absolutely unattainable for humans at this point.

An interesting study [10] reports cytokine profiles of 19 patients with MS, including 16 patients with RRMS, 1 individual with PPMS, and 2 — with SPMS. The patients were distributed into groups based on disease duration from the moment of diagnosis: 4.2 ± 0.8 months in group 1 and 76.6 ± 14.3 months in group 2. The study showed that in earlier stages of MS (in comparison with later stages and the absence of the disease), interferon gamma (IFNγ) and the anti-inflammatory lymphokine IL-10 dominate in the cytokine profiles. In the late stage, the levels of IL-1RA, IL-8, IL-12(p70), CCL-3, CCL-7, CCL-11, CXCL-10, FGF, and IFNγ go down. Later stages are also characterized by elevated levels of IL-1a, IL-1b, IL-2RA, IL-3, IL-4, IL-7, IL-12(p40), IL-18, CCL-5 (RANTES), CCL-27, HGF, MIF, M-CSF and TRAIL. Interestingly, MS patients were shown to have elevated blood levels of IL-17, known to play a key role in triggering development of psoriatic skin lesions [11]. In addition, patients with RRMS exhibited elevated IL-22 levels. Dynamics of cytokine profiles in the cerebrospinal fluid drove the researchers [10] to the conclusion about the crucial role of the accumulating IFNγ and MIF (a key factor of joint capsule degeneration in osteoarthritis) and a few other factors stimulating migration of lymphocytes: CCL-5 (RANTES), CCL-2 and CCL-27, induced by IFNγ and MIF. The study also revealed accumulation of proapoptotic TNFα and TRAIL- ligand in the cerebrospinal fluid (but not blood) of MS-stricken patients.

These data suggest a few patterns typical for MS, including increased long-term systemic activity of hematopoietic growth factors, in particular those targeting granulocytes, sustained Th1-response, and overrepresentation of lymphocyte/ monocyte migration factors in the absence of pronounced proinflammatory response (factors stimulating production and taxis of neutrophils). The study [10] could provide an insight into how cytokine levels observed in the cerebrospinal fluid and blood change in patients with MS, but due to the limitations of the applied statistical methods, significance of the identified patterns is questionable.

Considering the above said, our study aimed to

  1. investigate the short-term dynamics of cytokines in rats with rapidly progressing induced EAE;
  2. compare the data on cytokine levels in patients with MS and in rats with induced EAE in order to assess the feasibility of the EAE rat model for testing anti-MS candidate drugs.

METHODS

Induction of EAE in rats

Experiments involving laboratory animals were carried out in compliance with the “Regulations for the use of Experimental Animals” (Addendum to Order 755 of the Ministry of Health of the USSR dated August 12, 1977) and the principles of the Declaration of Helsinki (2013).

Homogenates of the spinal cord of random-bred rats were prepared as described in [12]. Further in vivo experiments were carried out in Dark Agouti rats weighing 220–250 g. The main group included 11 animals. On day 0 the animals were injected with the spinal cord homogenate mixed with incomplete Freund’s adjuvant in the ratio of 1 : 1 into the hind footpads. The total volume of the injected mixture was 100 μl per paw. The controls (n = 7) received 100 μl of normal saline mixed with incomplete Freund’s adjuvant in the ratio of 1 : 1. From day 1 through day 7, except for day 6, blood samples were collected from the tail vein (500 μl of blood daily) and immediately used for serum preparation. Briefly, blood was placed into Vacuette Z serum sepclot activator vacuum test tubes and centrifuged for 15–20 min at 2,500 rpm and +4 °С. The obtained serum (about 100 μl) was transferred to microcentrifuge tubes and frozen at –20 °С. The animals were weighted daily, and the severity of the disease was assessed using the following scale: 0 points — no symptoms, 1 point — decreased tail tone, 2 points — impaired righting reflex, 3 points — partial paralysis, 4 points — complete paralysis, 5 points— moribund or dead. In borderline cases, a lower index value was opted. Clear signs of EAE appeared in the controls starting from day 8 to day 14 of the experiment. On days 11–14 the disease reached its peak, which lasted for 2–3 days.

Multiplex cytokine assay

Serum samples were analyzed on the Bio-Plex platform (Bio-Rad, USA) using the Bio-Plex Pro Rat Cytokine 24-plex Assay (Bio-Rad). This assay employs magnetic beads coated with monoclonal antibodies to rat cytokines. It was performed according to the manufacturer’s recommendations and the protocol published in [13]. Serum was divided into 50 μl aliquots for the analysis. Mean fluorescence intensity of each sample was measured on Luminex 200 analyzer (Luminex Corporation, USA). Data were processed using MasterPlex CT and MasterPlex QT analysis software (Hitachi Solutions America, USA). For each analyte a calibration curve was constructed using 7 concentrations expressed as pg per 1 ml serum.

Statistical analysis

Two quartiles and median values of cytokine levels in each group were calculated daily for each cytokine. Then, significance of differences between the groups was tested using the nonparametric Mann-Whitney test and Statistica 8.0 for Windows. At p-value > 0.05 the differences were considered insignificant; we also used 3 significance thresholds: p ≤ 0.05, p ≤ 0.01, and p ≤ 0.001.

RESULTS

The data on the short-term dynamics of cytokine levels in human and animal blood are still scarce. Multiplex assays are expensive, and daily blood tests in MS patients and lab animals can be technically challenging or raise ethical concerns. Data obtained from the controls in the course of our experiment demonstrate that although incomplete Freund’s adjuvant injected into the footpads does not induce EAE, it still causes considerable fluctuations of cytokine levels in animals’ blood, rendering less reliable the assessment of the impact of the spinal cord homogenate on the course of the disease. Therefore, special statistical methods are needed to analyze the dynamics of cytokine profiles.

All animals included in the main group developed paralysis of the hind legs. The rising phase of the disease was observed on days 11–13, while the decline — on days 12–17. By day 18 all animals had recovered from the paralysis. Blood was collected on days 1 through 7 in the absence of visible signs of EAE.

Tab. 1 and tab. 2 show that on day 1 of the experiment the levels of 13 of total 24 analytes (IL-1a, IL-2, IL-4, IL-5, IL-6, IL-7, IL-12(p70), IL-17, IL-18, G-CSF, IFN-γ, RANTES (CCL-5), and MCP-1 (CCL-2) ) were significantly higher (by up to 220 % for IL-4) in the main group than in the controls in terms of the second and third significance thresholds (fig. 1). On day 2 no significant differences were observed for all studied cytokines. On day 3 differences were observed for IL-1b and VEGF (≤ 0.05), but on day 4 again no differences were found. On day 5 the main group demonstrated a considerable decrease in the levels of IL-1a, IL-1b, IL-13, and erythropoietin (fig. 2). On day 7 the differences between the groups were observed for 14 of 24 studied cytokines. Those were practically the same cytokines that showed differences on day 1, although statistical significance was confirmed for IL-10 and erythropoietin GM- CSF only and was not confirmed for IL-12(p70) and G-CSF (fig. 3) Of note, the levels of 13 of 14 cytokines in the main group were higher than in the controls. The only exception was GM-CSF that dropped from 8.17 pg/ml to 2.00 pg/ml.

DISCUSSION

A cytokine burst on day 1 of the experiment followed by a drop on day 2 should be interpreted as a manifestation of acute clonal nonspecific response to excess myelin outside the CNS. The response to the myelin manifested as simultaneous release of several lymphoproliferative factors is likely to be stimulated by hyperproduction of IL-1b originating from macrophages, dendritic cells and skin fibroblasts.

Increased cytokine synthesis on days 5 and 7 is, most probably, the result of the step-by-step accumulation of various clonal-specific lymphocytes, including those with autologous reactivity to myelin. Such longitude of the reaction is typical for the systemic clonal expansion of T-cells and eventually leads to visible physiological symptoms.

The most significant differences between the main and the control groups on day 7 were observed for the levels of IL-18 (2,475.85/4,182.05 pg/ml), RANTES (756.78/1,310.78 pg/ml), МCP 1 (CCL 2) (1,909.68/3,300.50 pg/ml) and IL-2 (743.52/1,091.57 pg/ml). Considering that IL-2 has been proved to induce production of other growth and hematopoietic factors [14], an assumption can be made that IL-2 triggers synthesis of such nonspecific immune factors as VEGF and erythropoietin, as well as IL-13, whose synthesis lagged in phase with respect to IL-2. Considering persistently high levels of IL-2 typical for patients with MS [10], this lymphokine seems to play a key role in the mass proliferation of lymphocytes outside the CNS. Increasing levels of lymphoproliferative and hematopoietic IL-4, IL-5, IL-6, Il-7, and IL-13 in the backdrop of decreased GM-CSF can be described as a cascade induced with IL-2 participation.

Unlike MS of humans, EAE in rats is not accompanied by production of proapoptotic TNFα, regardless of the increased synthesis of its classic inducers IL-12, IL-18 and IFNγ [14]. Therefore, elevated levels of TNFα in patients with MS are rather a result and not the cause of myelin destruction. At the same time, TNFα can contribute significantly to the damage of astrocytes and neurons in the late stages of MS.

According to the pattern described in [10], simultaneous increase and decrease of IFNγ and RANTES (CCL-5), respectively, in rats with EAE simulate similar processes occurring in humans with MS. The early stages of EAE in rats are not accompanied by an increase in GRO/KC (CXCL1) responsible for lymphocyte infiltration in the CNS, which renders the rat model different from MS in humans [10].

Both rats with EAE and humans with MS have hyperproduction of IL-17 which can contribute to the accumulation of specific lymphocytes in the CNS and activate their toxic function.

In spite of IL-1b hyperproduction, MS in humans shows no signs of neutrophil involvement in the pathology, which is also true for the factors regulating neutrophil taxis and activation. This pattern turned to be no different in the studied rat model.

The levels of M-CSF stimulating proliferation of neutrophil precursors did not change throughout the experiment. The same pattern was observed for MIP-3a (CCL20) that protects mucosa from bacterial infection and for leptin that raises body temperature in infected individuals.

Hyperproduction of IL-4 and IL-10 in rats with EAE in the background of elevated IL-5, IL-13, and GM-CSF should be considered a factor stimulating proliferation of B-cells. In theory, this set of cytokines can trigger synthesis of oligoclonal antibodies, but this effect has not yet been described in the literature.

Our experiment proves that proliferation of myelin-specific lymphocytes can be triggered outside the CNS. However, the course of EAE in rats and the course of MP in humans differ considerably. We cannot rule out that the first event occurring at the onset of the disease is infiltration of the CNS by lymphocytes that do not undergo clonal expansion but do undergo further selection in the presence of excess myelin. Abnormal behavior of lymphocytes observed in the rat model can be a result of their primary clonal-nonspecific hyperproliferation triggered by systemic or local excess of lymphoproliferative factors or/and lymphotaxis factors originating in CNS. Another possibility is induction of abnormally rapid degradation of myelin in CNS leading to a massive release of degradation products into the systemic circulation. In this case the rat model seems to be quite adequate to the early stages of MS in humans.

CONSLUSIONS

Data on the dynamics of cytokine production in rats with EAE obtained with the multiplex cytokine assay suggest that the rat model adequately imitates the course of MS in humans with respect to the levels of systemic lymphoproliferative and hematopoietic factors IL-1b, IL-2, IL-4, IL-5, IL-6 and IL-7. With respect to factors regulating taxis of lymphocytes, monocytes and other immune cells, the model fairly well imitates behavior of IL-17, RANTES (CCL-5) and MCP-1 (CCL-2), but exhibits a different dynamics for GRO/KC (CXCL1) levels. The model resembles the course of MS in humans in terms of IFNγ, IL-6 and IL-17 involved in cytotoxic and apoptotic reactions, but exhibits a different dynamics for TNFα.

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