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addition, expression plasmids have been used for so-called DNA immune response (and likely the innate response through NK
vaccines, which remain experimental due to issues with cost of cells) to viral infection, there have been many efforts to under-
production and necessity to inject the DNA. stand the physical configuration of this genetic region in other
A third general approach has been inactivated (killed or birds. Among the gallinaceous birds, the turkey, grouse and pheas-
fixed) viral vaccines, which do not infect cells but are taken ant MHC are extremely similar in all aspects to chickens, while
up by APCs for presentation and generally elicit only humoral the prairie chicken has but one classical class I gene (Wang et al.,
responses. Since the adaptive immune system will not even 2012; Eimes et al., 2013). Quails have a similar layout, but with
begin to respond without stimulation and direction of the innate several rounds of gene duplication. It was originally suggested
immune system through the PRRs, PAMPs or danger-associated that the many class I and class II B genes were an evolutionary
molecular patterns (DAMPs, such as oil–water emulsions) that response to greater pathogen challenges for migrating quails (and
act as vaccine adjuvants are generally included. Such inactivated this may still be true with specialized MHC molecules), but it has
vaccines do not pass across epithelial barriers, so typically they since been reported that there are single dominantly expressed
are injected, which is cumbersome and costly. However, they are class I and class II genes, just as in chickens (Shiina et al., 2004,
good for boosting antibody responses after an initial vaccination 2006).
with live viral vaccines, and therefore are used often for longer- In ducks, there are five class I genes in a row, with a single
lived egg-laying chickens. dominantly expressed class I gene (UAA) right next to the
An important delivery system that involves injection is in ovo polymorphic TAP genes (Mesa et al., 2004; Moon et al., 2005).
vaccination, in which eggs a few days before hatching are inocu- The expression of another classical class I gene (UAC) is under
lated. The development of a commercially available machine microRNA control and increases during infection (Chan et al.,
capable of injecting tens of thousands of eggs an hour has reduced 2016). Moreover, there are indications that the dedicated chap-
costs to the point that in ovo vaccination is widely used for broiler erone tapasin is missing in ducks (Magor et al., 2013). Several
chickens in some countries (for instance, North America). How- birds including falcons are reported to have a single dominantly
ever, the immune system of late stage eggs seems to respond expressed class I molecule (Alcaide et al., 2013), while the class
far better to live attenuated and viral vectored vaccines than to II regions of parrots are similar to the chicken, except that a class
inactivated vaccines. There also can be difficulties with maternally II A gene is next to the class II B genes (Hughes et al., 2008), as
transmitted antibody, in which antibodies from a vaccinated hen in mammals. A rather different organization has been suggested
are passed to the egg and can therefore block vaccine responses for ibis and oriental stork, but the assemblies are based on short
in chicks. sequencing reads and are incomplete, so the interpretations
While vaccines are a mainstay in protection from viral disease, remain speculative (Chang et al., 2015; Chen et al., 2016; Tsuji
they can also select for viruses that escape immunity, if they et al., 2017).
protect only against disease and not against transmission of the About half of all bird species are passerines (perching and
pathogen between hosts. For instance, vaccination leads to the song birds), and here the situation is even less clear. Early work
selection of antigenic variants for AIV and IBDV, more virulent highlighted the number of class I sequences expressed by great
viruses for IBDV and MDV, and changes in cell tropism for MDV reed warblers and willow warblers (Westerdahl et al., 2000).
(Nagarajan and Kibenge, 1997; Barrow et al., 2003; Davison and More recently, the same lab reports that three species of sparrows
Nair, 2005; Ingrao et al., 2013). The changes found in MDV viru- all have a number of classical (and non-classical) class I genes
lence has been used as the basis for generalizing the arguments for expressed, but only one at a high level (Drews et al., 2017). The
human medicine and public health (Read et al., 2015). zebra finch, studied extensively as a model organism for neuro-
biology and behaviour, is reported to have many class II B genes
Birds other than chickens located on various chromosomes, but only one classical class
Most of what is known about immunity to viruses outside of I gene located on a different chromosome from the TAP genes
mammals has been learned from chickens, but many birds suffer and class II B gene (Balakrishnan et al., 2010). However, genetic
important viral diseases. Other production and farm birds (tur- analysis shows that the classical class I gene is next to the class II B
keys, ducks and geese), sport birds (pigeons, falcons, quail and genes (Ekblom et al., 2011), so more careful research is required.
grouse), companion birds (parakeets/budgerigars) and wild birds Multiple class I and/or class II B genes have been reported for
(including a wide range of waterbirds and passerines) are known other passerine species (O’Connor et al., 2016), but in general
to (or must) suffer a variety of viral infections, and yet there is only there is little or no information on expression, so there is yet
limited knowledge of their immune responses (Adams et al., 2009; much to be done.
Wei et al., 2013; Q. Xu et al., 2016 ). Recent important challenges In terms of genetic control of responses to viral infections,
(among many others) include avian influenza virus, West Nile there are many observations that the MHC as defined by par-
virus and the psittacine beak and feather disease virus, and focus ticular genes correlates with responses to particular pathogens,
interest in further research, in part because of zoonotic potential typically avian malaria. Thus far, there are no reports linking pas-
(Reed et al., 2003; Pello and Olsen, 2013; Pérez-Ramírez et al., serine MHC responses to viral infection. There is also no work
2014; Staley and Bonneaud 2015; Fogle et al., 2016; Mostafa et on the adaptive immune responses, such as antibodies and T-cell
al., 2018). responses, to viral infection in passerines. Clearly, there is exciting
Given the central role of the chicken MHC in the adaptive research to be done.