Volume 2018, Issue 1 1019242
Research Article
Open Access

A Fractional Order Model for Viral Infection with Cure of Infected Cells and Humoral Immunity

Adnane Boukhouima

Adnane Boukhouima

Laboratory of Analysis, Modeling and Simulation (LAMS), Faculty of Sciences Ben M’sik, Hassan II University, P.O Box 7955 Sidi Othman, Casablanca, Morocco univh2c.ma

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Khalid Hattaf

Corresponding Author

Khalid Hattaf

Laboratory of Analysis, Modeling and Simulation (LAMS), Faculty of Sciences Ben M’sik, Hassan II University, P.O Box 7955 Sidi Othman, Casablanca, Morocco univh2c.ma

Centre Régional des Métiers de l’Education et de la Formation (CRMEF), 20340 Derb Ghalef, Casablanca, Morocco crmef-casablanca.net

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Noura Yousfi

Noura Yousfi

Laboratory of Analysis, Modeling and Simulation (LAMS), Faculty of Sciences Ben M’sik, Hassan II University, P.O Box 7955 Sidi Othman, Casablanca, Morocco univh2c.ma

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First published: 02 December 2018
Citations: 3
Guest Editor: Nurcan B. Savaşaneril

Abstract

In this paper, we study the dynamics of a viral infection model formulated by five fractional differential equations (FDEs) to describe the interactions between host cells, virus, and humoral immunity presented by antibodies. The infection transmission process is modeled by Hattaf-Yousfi functional response which covers several forms of incidence rate existing in the literature. We first show that the model is mathematically and biologically well-posed. By constructing suitable Lyapunov functionals, the global stability of equilibria is established and characterized by two threshold parameters. Finally, some numerical simulations are presented to illustrate our theoretical analysis.

1. Introduction

The immune response plays an important role to control the dynamics of viral infections such as human immunodeficiency virus (HIV), hepatitis B virus (HBV), hepatitis C virus (HCV), and human T-cell leukemia virus (HTLV). Therefore, many mathematical models have been developed to incorporate the role of immune response in viral infections. Some of these models considered the cellular immune response mediated by cytotoxic T lymphocytes (CTL) cells that attack and kill the infected cells [15] and the others considered the humoral immune response based on the antibodies which are produced by the B-cells and are programmed to neutralize the viruses [611]. However, all these models have been formulated by using ordinary differential equations (ODEs) in which the memory effect is neglected while the immune response involves memory [12, 13].

Fractional derivative is a generalization of integer derivative and it is a suitable tool to model real phenomena with memory which exists in most biological systems [1416]. The fractional derivative is a nonlocal operator in contrast to integer derivative. This means that if we want to compute the fractional derivative at some point t = t1, it is necessary to take into account the entire history from the starting point t = t0 up to the point t = t1. For these reasons, modeling some real process by using fractional derivative has drawn attention of several authors in various fields [1722]. In biology, it has been shown that the fractional derivative is useful to analyse the rheological proprieties of cells [23]. Furthermore, it has been deduced that the membranes of cells of biological organism have fractional order electrical conductance [24]. Recently, much works have been done on modeling the dynamics of viral infections with FDEs [2531]. These works ignored the impact of the immune response and the majority of them deal only with the local stability.

In some viral infections, the humoral immune response is more effective than cellular immune response [32]. For this reason, we improve the above ODE and FDE models by proposing a new fractional order model that describes the interactions between susceptible host cells, viral particles, and the humoral immune response mediated by the antibodies; that is,
()
where x(t), l(t), y(t), v(t), and w(t) are the concentrations of susceptible host cells, latently infected cells (infected cells which are not yet able to produce virions), productive infected cells, free virus particles, and antibodies at time t, respectively. Susceptible host cells are assumed to be produced at a constant rate λ, die at the rate dx, and become infected by virus at the rate f(x, v)v. Latently infected cells die at the rate ml and return to the uninfected state by loss of all covalently closed circular DNA (cccDNA) from their nucleus at the rate ρl. Productive infected cells are produced from latently infected cells at the rate γl and die at the rate ay. Free virus particles are produced from productive infected cells at the rate ky, cleared at the rate μv, and are neutralized by antibodies at the rate qvw. Antibodies are activated against virus at the rate gvw and die at the rate hw.
In system (1), Dα represents the Caputo fractional derivative of order α defined for an arbitrary function φ by
()
with 0 < α ≤ 1 [33]. Further, the infection transmission process in (1) is modeled by Hattaf-Yousfi functional response [34] which was recently used in [35, 36] and has the form f(x, v) = βx/(α0 + α1x + α2v + α3xv), where α0, α1, α2, α3 ≥ 0 are the saturation factors measuring the psychological or inhibitory effect and β > 0 is the infection rate. In addition, this functional response generalizes many common types existing in the literature such as the specific functional response proposed by Hattaf et al. in [37] and used in [2, 31] when α0 = 1; the Crowley-Martin functional response introduced in [38] and used in [39] when α0 = 1 and α3 = α1α2; and the Beddington-DeAngelis functional response proposed in [40, 41] and used in [3, 4, 10] when α0 = 1 and α3 = 0. Also, the Hattaf-Yousfi functional response is reduced to the saturated incidence rate used in [9] when α0 = 1 and α1 = α3 = 0 and the standard incidence function used in [27] when α0 = α3 = 0 and α1 = α2 = 1, and it was simplified to the bilinear incidence rate used in [5, 6] when α0 = 1 and α1 = α2 = α3 = 0.

On the other hand, system (1) becomes a model with ODEs when α = 1, which improves and generalizes the ODE model with bilinear incidence rate [42], the ODE model with saturated incidence rate [43], and the ODE model with specific functional response [44].

The rest of the paper is organized as follows. The next section deals with some basic proprieties of the solutions and the existence of equilibria. The global stability of equilibria is established in Section 3. To verify our theoretical results, we provide some numerical simulations in Section 4, and we conclude in Section 5.

2. Basic Properties and Equilibria

In this section, we will show that our model is well-posed and we discuss the existence of equilibria.

Since system (1) describes the evolution of cells, then we need to prove that the cell numbers should remain nonnegative and bounded. For biological considerations, we assume that the initial conditions of (1) satisfy
()
Then we have the following result.

Theorem 1. Assume that the initial conditions satisfy (3). Then there exists a unique solution of system (1) defined on [0, +. Moreover, this solution remains nonnegative and bounded for all t ≥ 0.

Proof. First, system (1) can be written as follows:

()
where
()

It is important to note that when α = 1, (4) becomes a system with ODEs. In this case, we refer the reader to [45] for the existence of solutions and to the works [4650] for the stability of equilibria. In the case of FDEs, we will use Lemma 2.4 in [31] to prove the existence and uniqueness of solutions. Hence, we put

()
We discuss four cases:
  • (i)

    If α0 ≠ 0, F(X) can be formulated as follows:

    ()

    • where

      ()

    • Hence,

      ()

  • (ii)

    If α1 ≠ 0, we can write F(X) in the form

    ()

    • where

      ()

    • Moreover, we get

      ()

  • (iii)

    If α2 ≠ 0, we have

    ()

    • where

      ()

    • Further, we obtain

      ()

  • (iv)

    If α3 ≠ 0, we have

    ()

    • where

      ()

    • Then

      ()

Hence, the conditions of Lemma 2.4 in [31] are verified. Then system (1) has a unique solution on [0, +. Now, we show the nonnegativity of solutions. By (1), we have
()
As in [31, Theorem 2.7], we deduce that the solution of (1) is nonnegative.

Finally, we prove the boundedness of solutions. We define the function

()
Then, we have
()
where δ = min⁡{d, m, a/2, μ, h}. Thus, we obtain
()
Since 0 ≤ Eα(−δtα) ≤ 1, we get
()
This completes the proof.

Now, we discuss the existence of equilibria. It is clear that system (1) has always an infection-free equilibrium E0(λ/d, 0,0, 0,0). Then the basic reproduction number of (1) is as follows:
()
To find the other equilibria, we solve the following system:
()
()
()
()
()
From (29), we get w = 0 or v = h/g. Then we discuss two cases.
If w = 0, by (25)-(28), we have l = (λdx)/(m + γ), y = γ(λdx)/a(m + γ), v = kγ(λdx)/aμ(m + γ), and
()
Since l ≥ 0, y ≥ 0, and v ≥ 0, then xλ/d. Consequently, there is no equilibrium when x > λ/d.
We define the function h1 on [0, λ/d] by
()
We have h1(0) = −aμ(m + ρ + γ)/kγ < 0, , and h1(λ/d) = (aμ(m + ρ + γ)/kγ)(R0 − 1).

Hence if R0 > 1, (30) has a unique root x1 ∈ (0, λ/d). As a result, when R0 > 1 there exists an equilibrium E1(x1, l1, y1, v1, 0) satisfying x1 ∈ (0, λ/d), l1 = (λdx1)/(m + γ), y1 = γ(λdx1)/a(m + γ), and v1 = kγ(λdx1)/aμ(m + γ).

If w ≠ 0, then v = h/g. By (25)-(27), we obtain l = (λdx)/(m + γ), y = γ(λdx)/a(m + γ), w = kγg(λdx)/aqh(m + γ) − μ/q, and
()
Since l ≥ 0, y ≥ 0, and w ≥ 0, we have xλ/dahμ(m + γ)/dkgγ. Hence, there is no equilibrium if x > λ/dahμ(m + γ)/dkgγ.
We define the function h2 on [0, λ/dahμ(m + γ)/dkgγ] by
()
We have h2(0) = −gλ(m + ρ + γ)/h(m + γ) < 0, , and h2(λ/dahμ(m + γ)/dkgγ) = h1(λ/dahμ(m + γ)/dkgγ).
Let us introduce the reproduction number for humoral immunity as follows:
()
which 1/h denotes the average life expectancy of antibodies and v1 is the number of free viruses at E1. For the biological significance, R1 represents the average number of the antibodies activated by virus.
If R1 < 1, we have x1 > λ/dahμ(m + γ)/dkgγ and
()
Therefore, there is no equilibrium when R1 < 1.
If R1 > 1, then x1 < λ/dahμ(m + γ)/dkgγ and
()
In this case, (32) has one root x2 ∈ (0, λ/dahμ(m + γ)/dkgγ). Consequently, when R1 > 1, there exists an equilibrium E2(x2, l2, y2, v2, w2) satisfying x2 ∈ (0, λ/dahμ(m + γ)/dkgγ), l2 = (λdx2)/(m + γ), y2 = γ(λdx2)/a(m + γ), v2 = h/g, and w2 = kγg(λdx2)/aqh(m + γ) − μ/q. When R1 = 1, E1 = E2.

We summarize the above discussions in the following theorem.

Theorem 2.

  • (i)

    If R0 ≤ 1, then system (1) has one infection-free equilibrium of the form E0(x0, 0,0, 0,0), where x0 = λ/d.

  • (ii)

    If R0 > 1, then system (1) has an infection equilibrium without humoral immunity of the form E1(x1, l1, y1, v1, 0), where x1 ∈ (0, λ/d), l1 = (λdx1)/(m + γ), y1 = γ(λdx1)/a(m + γ), and v1 = kγ(λdx1)/aμ(m + γ).

  • (iii)

    If R1 > 1, then system (1) has an infection equilibrium with humoral immunity of the form E2(x2, l2, y2, v2, w2), where x2 ∈ (0, λ/dahμ(m + γ)/dkgγ), l2 = (λdx2)/(m + γ), y2 = γ(λdx2)/a(m + γ), v2 = h/g, and w2 = kγg(λdx1)/aqh(m + γ) − μ/q.

3. Global Stability of Equilibria

In this section, we focus on the global stability of equilibria.

Theorem 3. If R0 ≤ 1, then the infection-free equilibrium E0 is globally asymptotically stable and it becomes unstable if R0 > 1.

Proof. The proof of the first part of this theorem is based on the construction of a suitable Lyapunov functional that satisfies the conditions given in [51, Lemma 4.6]. Hence, we define a Lyapunov functional as follows:

()
where Φ(x) = x − 1 − ln(x) for x > 0. It is not hard to show that the functional L0 is nonnegative. In fact, the function Φ has a global minimum at x = 1. Consequently, Φ(x) ≥ 0 for all x > 0.

Calculating the fractional derivative of L0(t) along solutions of system (1) and using the results in [52], we get

()
Using λ = dx0, we obtain
()
Hence if R0 ≤ 1, then DαL0(t) ≤ 0. In addition, the equality holds if and only if x = x0, l = 0, y = 0, w = 0, and (R0 − 1)v = 0. If R0 < 1, then v = 0. If R0 = 1, from (1), we get f(x0, v)v = 0 which implies that v = 0. Consequently, the largest invariant set of is the singleton {E0}. Therefore, by the LaSalle’s invariance principle [51], E0 is globally asymptotically stable.

The proof of the instability of E0 is based on the computation of the Jacobean matrix of system (1) and the results presented in [5355]. The Jacobean matrix of (1) at any equilibrium E(x, l, y, v, w) is given by

()
We recall that E is locally asymptotically stable if the all eigenvalues ξi of (40) satisfy the following condition [5355]:
()
From (40), the characteristic equation at E0 is given as follows:
()
where
()
Obviously, (42) has the roots ξ1 = −d and ξ2 = −h. If R0 > 1, we have g0(0) = aμ(m + ρ + γ)(1 − R0) < 0 and limξ→+g0(ξ) = +. Then, there exists ξ > 0 satisfying g0(ξ) = 0. In addition, we have |arg(ξ)| = 0 < απ/2. Consequently, when R0 > 1, E0 is unstable.

Theorem 4.

  • (i)

    The infection equilibrium without humoral immunity E1 is globally asymptotically stable if R0 > 1, R1 ≤ 1, and

    ()

  • (ii)

    When R1 > 1, E1 is unstable.

Proof. Define a Lyapunov functional as follows:

()
Calculating the fractional derivative of L1(t), we get
()
Using λ = dx1 + (m + γ)l1, f(x1, v1)v1 = (m + ρ + γ)l1, γl1 = ay1, ky1 = μv1, and 1 − f(xi, vi)/f(x, vi) = ((α0 + α2vi)/(α0 + α1xi + α2vi + α3xivi))(1 − xi/x)∀ i ∈ {1,2}, we obtain
()
Hence,
()
Using the arithmetic-geometric inequality, we have
()
Since R1 ≤ 1, we have DαL1(t) ≤ 0 if dx1ρl1. It is easy to see that this condition is equivalent to (44). Furthermore, DαL1(t) = 0 if and only if x = x1, l = l1, y = y1, v = v1, and (R1 − 1)w = 0. We discuss two cases: If R1 < 1, then w = 0. If R1 = 1, from (1), we get Dαv1 = 0 = ky1μv1qv1w, and then w = 0. Hence, the largest invariant set of is the singleton {E1}. By the LaSalle’s invariance principle, E1 is globally asymptotically stable.

At E1, the characteristic equation of (40) is given as follows:

()
where
()
We can easily see that (50) has the root ξ1 = gv1h. Then, when R1 > 1, we have ξ1 > 0. In this case, E1 is unstable.

Theorem 5. The infection equilibrium with humoral immunity E2 is globally asymptotically stable if R1 > 1 and

()

Proof. Consider the following Lyapunov functional:

()
Computing the fractional derivative of L2(t) and using λ = dx2 + (m + γ)l2, f(x2, v2)v2 = (m + ρ + γ)l2, γl2 = ay2, ky2 = (μ + qw2)v2, and v2 = h/g, we get
()
From (49), we have DαL2(t) ≤ 0 when dx2ρl2. This condition is equivalent to (52). In addition, DαL2(t) = 0 if x = x2, l = l2, y = y2, and v = v2. Further, Dαv2 = 0 = ky2μv2qv2w; then w = w2. Consequently, the largest invariant set of is the singleton {E2}. By the LaSalle’s invariance principle, E2 is globally asymptotically stable.

It is important to note that when ρ is sufficiently small or γ is sufficiently large, the two conditions (44) and (52) are satisfied. Then, we have the following corollary.

Corollary 6. Assume that R0 > 1. When ρ is sufficiently small or γ is sufficiently large, then we have the following:

  • (i)

    The infection equilibrium without humoral immunity E1 is globally asymptotically stable if R1 ≤ 1.

  • (ii)

    The infection equilibrium with humoral immunity E2 is globally asymptotically stable if R1 > 1.

4. Numerical Simulations

In this section, we validate our theoretical results to HIV infection. Firstly, we take the parameter values as shown in Table 1.

Table 1. Parameter values of system (1).
parameters values parameters values parameters values
λ 10 a 0.27 h 0.2
d 0.0139 γ 0.01 g 0.0001
β 0.00024 k 800 α0 1
ρ 0.01 μ 3 α1 0.1
m 0.0347 q 0.01 α2 0.01
α3 0.00001

By calculation, we have R0 = 0.4274 ≤ 1. Then system (1) has an infection-free equilibrium E0(719.4245,0, 0,0, 0). By Theorem 3, the solution of (1) converges to E0 (see Figure 1). Consequently, the virus is cleared and the infection dies out.

Details are in the caption following the image
Stability of the infection-free equilibrium E0.
Now, we choose β = 0.0012 and we keep the other parameter values. Hence, we obtain R0 = 2.137, R1 = 0.8334, and
()
Consequently, condition (44) is satisfied. Therefore, the infection equilibrium without humoral immunity E1(176.6853,168.7712,6.2508,1666.9,0) is globally asymptotically stable. Figure 2 demonstrates this result. In this case, the infection becomes chronic.
Details are in the caption following the image
Stability of the infection equilibrium without humoral immunity E1.

Next, we take g = 0.0004 and do not change the other parameter values. In this case, we have R1 = 3.3338, ρβh = 0.0000024, and d(m + ρ + γ)(α0g + α2h) + ρλ(α1g + α3h) = 0.000006. Hence, condition (52) is satisfied. Consequently, system (1) has an infection equilibrium with humoral immunity E2(423.4261,92.0442,3.4090,500,245.4473) which is globally asymptotically stable. Figure 3 illustrates this result. We can observe that the activation of the humoral immune response increases the healthy cells and decreases the productive infected cells and viral load to a lower levels but it is not able to eradicate the infection.

Details are in the caption following the image
Stability of the infection equilibrium with humoral immunity E2.

5. Conclusion

In the present paper, we have studied the dynamics of a viral infection model by taking into account the memory effect represented by the Caputo fractional derivative and the humoral immunity. We have proved that the solutions of the model are nonnegative and bounded which assure the well-posedness. We have shown that the proposed model has three infection equilibriums, namely, the infection-free equilibrium E0, the infection equilibrium without humoral immunity E1, and the infection equilibrium with humoral immunity E2. By constructing suitable Lyapunov functionals, the global stability of these equilibria is fully determined by two threshold parameters R0 and R1. More precisely, when R0 ≤ 1, E0 is globally asymptotically stable, whereas if R0 > 1, it becomes unstable and another equilibrium point appears, that is, E1, which is globally asymptotically stable whenever R1 ≤ 1 and condition (44) is satisfied. In the case that R1 > 1, E1 becomes unstable and there exists another equilibrium point E2 which is globally asymptotically stable when condition (52) is satisfied. In addition, we remarked that when ρ is sufficiently small or γ is sufficiently large, conditions (44) and (52) are verified, and then the global stability of E1 and E2 is characterized only by R0 and R1.

From our theoretical and numerical results, we deduce that the order of the fractional derivative α has no effect on the dynamics of the model. However, when the value of α decreases (long memory), the solutions of our model converge rapidly to the steady states (see Figures 13). This behavior can be explained by the memory term 1/Γ(1 − α)(tu) α included in the fractional derivative which represents the time needed for the interaction between cells and viral particles and the time needed for the activation of humoral immune response. In fact, the knowledge about the infection and the activation of the humoral immune response in an early stage can help us to control the infection.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

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