The Origins and Development of Lymphocyte Populations. In: Porter R, Knight J, editors. Ciba Foundation Symposium 5 - Ontogeny of Acquired Immunity. Chichester: John Wiley & Sons, Ltd.; 1972. pp. 35-64. [DOI: 10.1002/9780470719886.ch3]

Bacterial interference with immunospecific defences

There are many descriptions of bacterial agents that can suppress different phases of specific immunity. Bacterial agents are frequently employed as tools to modulate the immune system in experimental models or clinical therapy. Little attention has been given to the implications of such immunoregulating properties for infectious processes or for the natural role of the microbial flora in the normal regulation of the immune system. There are very few studies attempting to investigate directly the possible role of bacterial immunosuppressant factors in acute infection. Several examples of natural and experimental chronic infections have been described in which progressive uncontrolled infection is associated with a depressed cell mediated immunity, but a causal relation is unproven. This depression is usually not antigenically specific and there is some evidence that it may be initiated and maintained by persistent cell-wall components of the microorganisms. The bacteria implicated in suppression are all facultative or obligate intracellular parasites that can multiply and survive in the monocyte/macrophage. Bacterial immunosuppressants may have a crucial role in the pathogenesis of progressive chronic inflammation that occurs after infection.

Some endocrine aspects of the thymus gland

In studies of the mouse thymus, lymphocyte mitoses are seen to be most frequent in the thymus cortex. There is evidence from thymic grafts that a hypothetical factor, thymopoietin, may stimulate mitosis of thymic lymphocytes. It is a factor which is postulated to act in conjunction with the PAS-positive mesenchymal reticular cells and epithelial reticular cells of the cortex. The thymus medulla is necessary for the integrity of thymic grafts, and may also elaborate a secretion for maintaining the cellular functions of the gland. Thymectomy has been used as a gauge for judging normal thymic function and results, in the mouse, in lymphopenia, degeneration of spleen and lymph nodes, delayed rejection of skin allografts, reduced ability of spleen cells to mount the graft versus host reaction, and reduced primary immune response to certain antigens. Correction of these deficiencies offers a means of evaluating various thymic extracts and grafts. Lymphocytosis-stimulating hormone (LSH) is known to maintain the peripheral lymphoid organs and cause lymphocytosis in the thymectomized animal. Diffusion chamber studies of thymic grafts also show restored lymphoid tissue by a cell-free factor (CIF). These two factors may be the same and probably represent the basis of the highly purified lymphocyte-stimulating proteins, LSHr and LSHh, which restore the L/P ratio in thymectomized animals and may stimulate lymphopoiesis in spleen and lymph nodes. LSHr, unlike LSHh, increases the total lymphocyte count. LSHr has been found to increase the humoral antibody response in neonatal mice both by the PFC technique and by direct hemolysis of sheep erythrocytes. Homeostatic thymic hormone (HTH) is a thymic extract of small molecular weight and contains nucleic acid. In the thymectomized guinea pig it has been found to maintain normal levels of lymphocytes in the blood, spleen and lymph nodes, to restore antibody titers to typhoid H antigen and to restore the toxic allergic reaction. Thymic humoral factor (THF) is of smaller molecular weight (less than 1,000) and probably is not a protein. It also enhances lymphoid proliferation in neonatally thymectomized mice. There is evidence that THF participates in humoral antibody formation because it stimulates PFC formation from neonatally thymectomized mice after inoculation with sheep erythrocytes. Its effects on cell-mediated immunity are seen from findings that injection of THF restores the ability of thymectomized mice to reject skin allografts. THF enables spleen cells from thymectomized or neonatal animals to mount the graft versus host reaction, and causes maturation of bone marrow cells and spleen or lymph node cells so that they can participate in the graft versus host reaction. It has been reported to stimulate lymphocytes to kill isogeneic tumor cells in vitro. Thymosin is protein extracted from the thymus. It has been found to alleviate leukopenia slightly and provide some improvement in lymphoid histology in thymectomized mice...