# American Institute of Mathematical Sciences

## Qualitative analysis of a simple tumor-immune system with time delay of tumor action

 1 Department of Mathematics, Shaanxi University of Science and Technology, Xi'an, 710021, China 2 Department of Immunology, Xi'an Medical University, Xi'an 710021, China 3 Center for Applied Mathematics, Guangzhou University, Guangzhou 510006, China, Department of Mathematical Sciences, University of Alabama in Huntsville, Huntsville, AL 35899, USA

* Corresponding author: Jianquan Li; ** Equal contributor: Dian Zhang

Received  May 2019 Revised  October 2020 Published  November 2020

Fund Project: Jianquan Li is supported by the National Natural Science Fund of China (Nos. 11971281, 12071268), Dian Zhang is supported by the Project of Xi'an Medical University (No. 2018GJFY05), Jia Li is supported by the National Natural Science Fund of China (No. 12071095), Xiaolin Lin is supported by the National Natural Science Fund of China (No. 11771259)

In this paper, we propose a simple tumor-immune system model with time delay of tumor action, where two kinds of effects of the tumor cells (i.e. stimulation and neutralization) on the effector cells are considered. The local stability of the model is obtained by analyzing the characteristic equations of the model at the corresponding equilibria, the sufficient conditions on the global stability are found by applying the Fluctuation Lemma and constructing the different convergent sequences. The obtained results show that, compared to the results for the model without time delay, the time delay of tumor action can affect the stability of tumor equilibrium of the model as the stimulation effect of the tumor cells is strong enough, while the delay is harmless for the stability of tumor equilibrium under the neutralization of tumor cells. For the appropriate neutralization of tumor cells on effector cells, the bistability of the tumor free equilibrium and the stronger tumor equilibrium can appear. In the case of stimulation of tumor cells, the sufficiently large time delay can lead to the appearance of a stable periodic solution by Hopf bifurcation, and the numerical simulation illustrates that the amplitude of the periodic orbit increases with time delay. We also discuss the dependence of the tumor equilibrium and the time delay threshold, determining the stability of the tumor equilibrium, on tumor action. The related conditions determining dynamics of the model are expressed by certain formulae with biological meanings.

Citation: Jianquan Li, Xin Xie, Dian Zhang, Jia Li, Xiaolin Lin. Qualitative analysis of a simple tumor-immune system with time delay of tumor action. Discrete & Continuous Dynamical Systems - B, doi: 10.3934/dcdsb.2020341
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The stable periodic orbit of model (6) as ${\mu} K>\eta$ and $n{\sigma}<r\eta$. Here, $r = 2.5, K = 2, n = 0.8, {\sigma} = 0.5, {\mu} = 4$, $\eta = 1.5$ and ${\tau} = 0.3>0.1943 = {\tau}_0.$ Model (6) has a unique tumor equilibrium $P^*(0.3272, 2.6138)$, and a stable periodic orbit
">Figure 2.  The bifurcation diagram of model (6) with respect to the time delay ${\tau}$ when ${\mu} K> \eta$ and $n{\sigma}<r\eta$. Here, except for the time delay ${\tau}$, the values of other parameters are the same as those for Figure 1
The trajectories of $T = T(t)$ and $E = E(t)$ of model (6) corresponding to three delays ${\tau} = 1, 5, 10$ in case of $0<{\mu}<\frac{\eta}{K}$ and $n{\sigma}<r\eta$, respectively. Other parameter values are $r = 2.5, K = 2, n = 0.8, {\sigma} = 0.5, {\mu} = 0.4$ and $\eta = 1.5$
, where the left column is for ${\tau} = 0.3$, the right column is for ${\tau} = 2.0$, and the initial conditions of model (6) are the same in the two columns. The first row is the orbits of model (6) in the $T$-$E$ plane, the second row is the trajectories of $T = T(t)$, and the third row is those of $E = E(t)$">Figure 4.  The trajectories of model (6) with two delays ${\tau} = 0.3, 2.0$ in Case C4 of Table 1, where the left column is for ${\tau} = 0.3$, the right column is for ${\tau} = 2.0$, and the initial conditions of model (6) are the same in the two columns. The first row is the orbits of model (6) in the $T$-$E$ plane, the second row is the trajectories of $T = T(t)$, and the third row is those of $E = E(t)$
, and the regions $D_1$ and $D_2$ correspond to the stable and unstable cases of the tumor equilibrium $P^*$, respectively">Figure 5.  The graph of function ${\tau}_0 = {\tau}_0({\mu})$ defined by (16) with ${\mu} >\frac{\eta}{K}$. Here, except for ${\mu}$ and ${\tau}$, the values of other parameters are the same as those for Figure 1, and the regions $D_1$ and $D_2$ correspond to the stable and unstable cases of the tumor equilibrium $P^*$, respectively
The existence and the stability of all feasible equilibria of model (2) without time delay. Here, US stands for unstable, GAS-1 globally asymptotically stable in the first quadrant of the $T$-$E$ plane, GAS-2 globally asymptotically stable on the set $\{(T, E): T\ge 0, E>0\}$, LAS locally asymptotically stable, SN saddle-node
 Case Conditions $P_0$ $P^*$ $P_*$ $P^*_*$ C1 ${\mu}>0, n{\sigma}0, n{\sigma}\ge r\eta$ GAS-2 — — — C7 $\begin{array}{c} {\mu}<0, \eta+ {\mu} K< 0, \\n{\sigma}>-\frac{rK}{4{\mu}}( {\mu}-\frac{\eta}{K})^2 \end{array}$ GAS-2 — — —
 Case Conditions $P_0$ $P^*$ $P_*$ $P^*_*$ C1 ${\mu}>0, n{\sigma}0, n{\sigma}\ge r\eta$ GAS-2 — — — C7 $\begin{array}{c} {\mu}<0, \eta+ {\mu} K< 0, \\n{\sigma}>-\frac{rK}{4{\mu}}( {\mu}-\frac{\eta}{K})^2 \end{array}$ GAS-2 — — —
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