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In this paper, we study intra-host viral adaptation by antigenic cooperation - a mechanism of immune escape that serves as an alternative to the standard mechanism of escape by continuous genomic diversification and allows to explain a number of experimental observations associated with the establishment of chronic infections by highly mutable viruses. Within this mechanism, the topology of a cross-immunoreactivity network forces intra-host viral variants to specialize for complementary roles and adapt to the host's immune response as a quasi-social ecosystem. Here we study dynamical changes in immune adaptation caused by evolutionary and epidemiological events. First, we show that the emergence of a viral variant with altered antigenic features may result in a rapid re-arrangement of the viral ecosystem and a change in the roles played by existing viral variants. In particular, it may push the population under immune escape by genomic diversification towards the stable state of adaptation by antigenic cooperation. Next, we study the effect of a viral transmission between two chronically infected hosts, which results in the merging of two intra-host viral populations in the state of stable immune-adapted equilibrium. In this case, we also describe how the newly formed viral population adapts to the host's environment by changing the functions of its members. The results are obtained analytically for minimal cross-immunoreactivity networks and numerically for larger populations. • Antigenic cooperation is a mechanism of immune escape which provides an alternative to immune escape by continuous genomic diversification. • Intra-host viral variants create a network of cross-immunoreactivity, with each variant performing a different role to escape the host immune system. • Emergence of a new viral variant with altered antigenic features may result in a rapid rearrangement of the viral ecosystem and a change in the roles played by existing viral variants. • Emergence of a new viral variant with altered antigenic features may push the viral population into a stable state of immune escape by antigenic cooperation. • Viral transmission between two infected hosts results in the merging of two intra-host viral populations and results in changes in the functions of its members to adapt to the new host environment. [ABSTRACT FROM AUTHOR]

Immunological memory can be regarded as the key aspect of adaptive immunity, i.e. a specific response to first contact with an antigen, which in mammals is determined by the properties of T, B and NK cells. Re-exposure to the same antigen results in a more rapid response of the activated specific cells, which have a unique property that is the immunological memory acquired upon first contact with the antigen. Such a state of immune activity is also to be understood as related to "altered behavior of the immune system" due to genetic alterations, presumably maintained independently of the antigen. It also indicates a possible alternative mechanism of maintaining the immune state at a low level of the immune response, "directed" by an antigen or dependent on an antigen, associated with repeated exposure to the same antigen from time to time, as well as the concept of innate immune memory, associated with epigenetic reprogramming of myeloid cells, i.e. macrophages and NK cells. Studies on Teleostei have provided evidence for the presence of immunological memory determined by T and B cells and a secondary response stronger than the primary response. Research has also demonstrated that in these animals macrophages and NK-like cells (similar to mammalian NK cells) are able to respond when re-exposed to the same antigen. Regardless of previous reports on immunological memory in teleost fish, many reactions and mechanisms related to this ability require further investigation. The very nature of immunological memory and the activity of cells involved in this process, in particular macrophages and NK-like cells, need to be explained. This paper presents problems associated with adaptive and innate immune memory in teleost fish and characteristics of cells associated with this ability. • The presence of adaptive and innate immune memory has been confirmed in fish. • Adaptive immune memory in fish develops with the participation of T and B lymphocytes. • Innate immune memory is related to the properties of macrophages and NK-like cells. • There is a need to further verify the biological basis of this property. [ABSTRACT FROM AUTHOR]