RISH. VI: General evolutionary aspects of the immune system

Sexual selection by female immunity against paternal antigens can fix loss of function alleles

Humans lack the common mammalian cell surface molecule N-glycolylneuraminic acid (Neu5Gc) due to a CMAH gene inactivation, which occurred approximately three million years ago. Modern humans produce antibodies specific for Neu5Gc. We hypothesized that anti-Neu5Gc antibodies could enter the female reproductive tract and target Neu5Gc-positive sperm or fetal tissues, reducing reproductive compatibility. Indeed, female mice with a human-like Cmah(-/-) mutation and immunized to express anti-Neu5Gc antibodies show lower fertility with Neu5Gc-positive males, due to prezygotic incompatibilities. Human anti-Neu5Gc antibodies are also capable of targeting paternally derived antigens and mediate cytotoxicity against Neu5Gc-bearing chimpanzee sperm in vitro. Models of populations polymorphic for such antigens show that reproductive incompatibility by female immunity can drive loss-of-function alleles to fixation from moderate initial frequencies. Initially, the loss of a cell-surface antigen can occur due to drift in isolated populations or when natural selection favors the loss of a receptor exploited by pathogens, subsequently the same loss-of-function allele can come under sexual selection because it avoids being targeted by the female immune system. Thus, we provide evidence of a link between sexual selection and immune function: Antigenicity in females can select against foreign paternal antigens on sperm and rapidly fix loss-of-function alleles. Similar circumstances existed when the CMAH null allele was polymorphic in ancestral hominins, just before the divergence of Homo from australopithecines.

Invertebrate immunity: another viewpoint

All vertebrates and invertebrates manifest self/non-self recognition. Any attempt to answer the question of adaptive significance of recognition must take into account the universality of receptor-mediated responses. These may take two forms: (1) rearranging, clonally distributed antigen-specific receptors that distinguish in the broadest sense between self and non-self, and non-self A from non-self B, latecomers on the evolutionary scene; (2) pattern recognition receptors, the earliest to evolve and still around, necessitating the requirement for induced second signals in T- and B-cell activation. Either strategy need not force upon invertebrates the organization, structure and adaptive functions of vertebrate immune systems. Thus, we can freely delve into the unique aspects of the primitive immune mechanisms of invertebrates. In contrast, using the opposite strategy which is still problematic, i.e. linking invertebrate and vertebrate defence, seems to give us an approach to universality that might eventually reveal homologous kinship.

Immune response: tissue specific T-lymphocytes

The lymphatic system forms a 'blind' plexus of vessels that in general are found in tissue which has an inherently high replicative capacity. It is this system that is responsible for the rapid deployment and circulation of tissue-specific T-lymphocytes for the inspection of cell-surface aberrations within the tissue. The presence of tissue-specific T-lymphocytes explains why 90% of lymphocytes are found outside the lymphatic system and why they migrate in a selective manner. The tissue-specific T-lymphocyte is considered to express a common lymphocyte cell surface pattern, the homotype, and a tissue-specific cell-surface pattern, the histotype which may involve MHCA and mHCA. It is the histotypic pattern that is responsible for the tissue specificity of the tissue-specific T-lymphocyte. The presence of tissue-specific T-lymphocytes does pose problems for the immune system. If different tissue-specific T-lymphocytes met within a particular tissue, 'lost' lymphocytes, an immune response will be generated against the intruder (lost lymphocyte), and the intruder will not be able to recruit other immunocompetent cells in that tissue. This immune reaction is an attempt to change the histotypic pattern of the intruder. This situation would explain the autologous immune response. This response however is suppressed in the systemic system by immunosuppressive compounds from the liver. It is only in the tissues that the tissue-specific T-lymphocytes are released from this suppression, in order to initiate immune reactions against aberrant cell-surface patterns.

Immunology of pregnancy: towards a unifying hypothesis

The variable findings of hormonal-immunoregulation and the variable cellular and humoral immune responses in pregnancy have been considered in relationship to the physiological response. From such considerations it appears that the peripheral blood lymphocyte/leukocyte response in pregnancy is not important, but rather the local uterine immune response at implantation and throughout pregnancy. It is proposed, and evidence is presented, that a normal allogeneic immune response is initiated at the time of implantation of the blastocyst. This immune response regulates the invasive nature of the trophoblast and initiates the first stage of parturition. The initiation and maintenance of this immune response is based on an interplay between maternal and paternal HLA and trophoblast antigens. In the case of HLA-incompatible donor-recipient blastocyst transplants, a more pivotal role for immunoregulation by trophoblast antigens is proposed. This is because it is considered that the local uterine immune response suppresses the expression of allogeneic HLA. This concept is further developed in terms of haploid HLA suppression on maternal and fetal lymphocytes that cross the placenta. This is considered to allow the interaction of these lymphocytes with each other and explains maternal transfer of cell-mediated immunity.

Immune response: self-foreignness

The classical basic concept of the immune system as a defence system per se and immunological surveillance against neoplasia have never been satisfactorily verified experimentally. The reason for this lies in the historic development of immunology in terms of observations of infectious disease and the interpretation of those observations. Thus, based on a complete lack of understanding of immune events, immunization procedures were developed by Pasteur and his contemporaries. The success of some of these immunization methods, influenced by culture and philosophical thoughts, and based on prima facie evidence allowed the next conceptual step to be taken, culminating in the immune surveillance hypothesis. Central to this hypothesis is selection and tolerance to self-antigens. However, immune reactions to self-antigens are evident and clonal selection is not viable because the number of clones required increases as the frequency of chance of a cell belonging to a particular clone decreases. Also, circadian rhythms in the immune response have not been taken into account. In addition, the problems of haemocytopoiesis have not been addressed, in that it is possible for B-lymphocytes to become terminal macrophages and T-lymphocytes to become mast cells, eosinophils and/or basophils constituting 'dead end' cells in an immune response. The initiation of the immune response begins with a tissue-specific T-lymphocyte being stimulated and undergoes replication. This gives rise to a dual functional helper/suppressor cell and a B-lymphocyte. These basic concepts explain the necessity for auto-reactive lymphocytes, that is the autologous mixed lymphocyte reaction (AMR). The AMR is a natural consequence of having tissue-specific lymphocytes to monitor plasma membrane aberrations.