Tropical eosinophilia: a new look.
Tropical eosinophilia had probably been reported previously, perhaps before the turn of century, but the first identifiable description was by a Frenchman in a lecture delivered at Guy's Hospital in 1909. During the next three decades scattered reports from several tropical areas appeared in many scientific journals. In 1940, Frimodt-Moller through his classic paper, established tropical eosinophilia as a distinct entity.
Weingarten, in 1943, described his accidental discovery of arsenic as a specific therapeutic agent in tropical eosinophilia. Since then many other workers have confirmed the effectiveness of various organic arsenicals in this disease, and 50 to 98% of the cases have shown symptomatic relief. No figures are available as to the toxic effects of arsenicals as used in dosages in this condition. However the use of arsenicals in any form is always associated with potential hazards.
The specificity of diethylcarbamazine (a proved potent-antifilarial drug) in tropical eosinophilia was first proved by Ganatra and Lewis in 1955 and thereafter by Danaraj in 1956. Ganatra and Lewis treated 13 patients of tropical eosinophilia with diethylcarbamazine and complete clinical relief was observed within the first 20 days of the commencement of therapy. The clinical results with diethylcarbamazine compare well with those obtained with carbarsone (an arsenical). This was one of the important observations that focused attention on a possible filarial aetiology of tropical eosinophilia.
(i) Danaraj et al,  and Danaraj and Schacher, using alcoholic extracts of Diro-filaria immitis powder as an antigen, showed a positive complement fixation test in most patients with tropical eosinophilia. These observations indicate that tropical eosinophilia may well be a systemic manifestation of sensitivity to the filarial antigen.
(ii) Webb et al, in 1960, showed the presence of microfilariae in the lungs, liver and lymph nodes of patients satisfying all the diagnostic criteria of tropical eosinophilia, and showing good therapeutic responses to diethylcarbamazine. The microfilariae were found to be sheathed and showed the anatomical features characteristic of W. Bancrofti. In 1966, Danaraj et al also reported finding microfilariae in the sections of lung biopsies in 4 out of 5 patients with tropical eosinophilia. They also reviewed sections of the lungs from an earlier reported case and demonstrated degenerating segments of microfilariae. Udwadia in 1975, has given an account of 33 cases of tropical eosinophilia in whom open lung biopsies were done. Ten patients showed presence of microfilariae whereas in two of thirty-three patients, unidentified parasitic segments were observed.
(iii) Topographical distribution of endemic filariasis in India coincides with the incidence of tropical eosinophilia. Filariasis is common along the greater part of the Western coast of India particularly the coastal strip around Bombay, Goa and Kerala, along the whole of the Eastern coast of India, in Bengal, Bihar, Orissa, parts of Mysore and Andhra Pradesh as also in parts of Madhya Pradesh and Uttar Pradesh. Tropical eosinophilia is also very common in all the above mentioned parts of the country. 
Sri Lanka is endemic for both tropical eosinophilia and filariasis. Many countries in the South-East Asia regions are endemic for both these diseases. The B. Malayi is endemic in most South East Asian countries (excepting Singapore where W. Bancrofti is endemic) and in all these countries reports of tropical eosinophilia are fairly frequent.
A survey of other areas and countries in the world where filariasis is endemic shows that in many such areas tropical eosinophilia is also in existence.
Some of the recent literature suggests eosinophilia as a consequence of host defense against parasite. The following are some of the current concepts outlining the role of eosinophils in this respect.
(i) Eosinophils are effectors or killer cells
The clear association between eosinophils and parasites has led many investigators to examine both the mechanism of parasite induced eosinophilia and the possibility that the eosinophil is an effecter cell in parasite destruction. The evidence for the eosinophil as an effecter cell in antibody dependent damage to schistosomula has been demonstrated in in-vitro systems. However, it should be rioted that eosinophilia can develop in patients with agammaglobulinemia and other immune deficiency syndromes. Thus this possibility still exists that eosinophilia represents a cellular response which may function in the absence of specific immune mechanism.
Using an in-vitro assay, normal human peripheral blood leukocytes were shown to release  Chromium from labelled immature schistosomes.8 When the leukocytes were separated into eosinophils, neutrophils or mononuclear cell preparations, K-cell (killer cell) activity was associated with the eosinophils. Addition of neutrophils or mononuclear cells to purified eosinophils was not associated with an increase in chromium release. This shows that other cell types had no detectable synergistic action, nor did they exert cytotoxic effect on their owns.6, 7
Further evidence for the eosinophil as an effecter cell in antibody dependent damage has been demonstrated in 'in vivo' studies in schistosoma infected mice. In 'Schistosoma mansoni' infection in mice partial immunity can be transferred by immune serum which contains specific antibody directed against the schistosomula. The protective effect of serum was reduced by prior treatment of the mice with antieosinophil serum but not with antisera directed against lymphocytes, monocytes or neutrophils. Therefore, the partial immunity transferred by serum would also appear to require the participation of the eosinophils.
These findings strongly support the previous observations that the eosinophil has some parasitidal properties.
(ii) Eosinophil adherence to parasites
Considering the role of eosinophils in the mechanism of host defense it is clear that intimate relationship must be established between this cell and its surroundings. This relationship necessarily involves eosinophil surfaces particularly membrane receptors.
A number of recent investigations have disclosed the presence of immunoglobulins and complement receptors on eosnophils., , , ,  Ottesen et al in their classic studies demonstrated a predominant role of eosinophil in leukocyte adherence to schistosomulae. Degranulation of adherent eosinophils on the surface of the schistosomules was frequently observed, and it seems reasonable to speculate that the same receptor bearing eosinophils found adhering to schistosomules might well be responsible for the cytotoxic function already ascribed to these cells.
Very low percentage of normal eosinophils had demonstrated IgG receptors and this was in marked contrast to the 80-90% of neutrophils with these receptors. In the eosinophilic patients with helminthic infestations the percentage of IgG receptor bearing eosinophils in the blood was increased significantly over that of normal individuals (p < 0.05) but still the level did not approach those found on the neutrophils.
(iii) Role of eosinophils in hypersensitivity reactions
The peripheral blood hypereosinophilia in states of parasitic invasion leads to an expanded total pool of eosinophils, which are predominantly located in the tissues. Tissue eosinophils are prominent at mucosal and cutaneous surfaces in a distribution similar to that of the mast cells and the lymphocytes which produce IgE., ,  Mast cell derived chemotactic factors which preferentially attract eosinophils in-vitro and in vivo have been defined,, , ,  suggesting that the interplay of these two cell types may have a role in the localization of eosinophils at tissue sites of parasitic invasion.
Eosinophils attracted to the sites of parasite invasion fulfils at least two roles: defending the sensitized host by directly damaging the parasites and containing the immediate hypersensitivity reaction, involved by antigens released from the parasites.
A recently appreciated defensive capability of eosinophils, which bridges the traditional immune response and special functions of the eosinophils is their IgG antibody dependant destruction of schistosomula and schistosome eggs as shown in [Fig. 1].
The eosinophils possess enzymes which are capable of specifically degrading mast cell mediators and thereby containing immediate type of hypersensitivity reaction to antigen derived from parasite [Fig. 2]., 
Thus eosinophils attracted by diverse specific chemotactic factors to the sites of parasitic invasion are able to degrade many of the mast cell mediators elaborated by immediate hypersensitivity response to parasitic antigen.
These studies show that the eosinophil can no longer be regarded as an innocent bystander in inflammatory responses, but rather as an effective and active participant.
Tropical eosinophilia is a syndrome characterized by a generalized tissue reaction in which pulmonary manifestations predominate. The presence of cough, breathlessness, wheeze, marked increase in peripheral eosinophilia, and eosinophilic pulmonary infiltrates all point to an underlying hypersensitivity reaction.
There are two main forms of hyper sensitivity reaction, immediate and delayed. The immediate or antibody mediated form appears rapidly and depends on the production of pharmacologically active mediator substances activated by antigen antibody interaction. The delayed or cell mediated form appears very slowly (usually after 24 hours) and depends on immunologically activated lymphoid cells, which, on reaction with antigen appear to release substances known as lymphokinases, having a variety of effects on other cells and effects on blood vessel permeability. Further classifications of these hypersensitivity reactions are as follows; three of which come under the heading of immediate antibody mediated reactions, type I, anaphylactic reactions, type II, cytolytic or cytotoxic reactions, and type III, toxic complex syndrome. The fourth type is the cell mediated delayed hypersensitivity form of reactions.
Evidences enlisted earlier from the various studies strongly suggest a filarial infection as the important underlying cause for this hypersensitivity reaction in the cases of tropical eosinophilia.
We have carried out some immunological studies to assess the role and nature of immune reactions in the cases of tropical eosinophilia using various parasitic antigens such as D. immitis (Dog filaria), A. suum, N. americanus and T. canis. Though our studies showed generalized increase in all immunoglobulins, especially IgE levels were markedly elevated in all patients. A generalized rise in all immunoglobulin levels points to an associated type III reaction. The alveolitis in tropical eosinophilia is thus due to a type III reaction. It is the severity of this reaction which determines the extent of pulmonary damage in tropical eosinophilia. The marked rise in the IgE levels points to a type I, reaction., ,  The bronchial spasm, mucosal oedema, increased secretion of mucus and sloughing of the mucous membrane within the bronchial lumen in tropical eosinophilia would thus be the result of immediate type of hypersensitivity [Table - 1].
When we tested for the specificity of IgE response by Rat Mast Cell Degranulation Technique (RMCT) against these parasitic antigens, we found that 11 of the nineteen subjects showed a specific response to D. immitis antigen. There was a cell mediated sensitization predominant to D. immitis antigen in 13 out of the twenty one subjects while the antigens of A. suum, N. americanus and T. canis showed values comparable to controls.
In the nineteen subjects in whom both cell mediated immune response as well as Rat Mast Cell Degranulation Test, to test the presence of specific IgE response of the host, were done, it was found that six patients, showed specific IgE response as judged by Rat Mast Cell Degranulation Technique to D. immitis but no cell mediated immune response to the same antigen. There were eight subjects who showed a specific cell mediated response to D. immitis but no specific IgE response and there were five patients who demonstrated cell mediated and IgE responses specific to D. immitis antigen. There were no cases where either of these responses were absent [Fig. 3].
Though both cell mediated and humoral responses occur simultaneously, one or the other response could predominate at any given time. These studies show that either IgE or cell mediated response could be more evident in some subjects or both were detectable in some others. This predominance of the immune response was not related to the severity of the symptoms or the duration of the disease.
These studies give a more definite evidence of an immune response occurring to a filarial antigen in patients with tropical eosinophilia. Earlier studies were indirect pointers i.e. serological tests and skin sensitivity which are not sensitive and specific indices. The more modern techniques of evaluating immune changes i.e. both humoral and CMI have given support to a hypothesis of filarial aetiology of tropical eosinophilia.
Marked eosinophilia is associated with worm infestations, allergies and neoplasm., ,  The increase in eosinophils in the above conditions is not usually accompanied by changes in the neutrophils or basophils, although all of the granulocytes are thought to develop from a common stem cell. This dissociation in the production of different granulocytes suggests the presence of specific regulatory mechanisms for each cell line. Specific stimuli for eosinophil chemotaxis and migration have been studied by several investigators., , ,  The central role of lymphocytes in the induction of the eosinophilic response after Trichineal spiralis infection was reported by Basten and Beeson. They postulated that the increased production of eosinophils under these conditions might be mediated by a diffusible factor. Miller et al, in 1976, described a diffusible stimulator of eosinophilopoiesis which is produced by spleen cells maintained in diffusion chambers implanted intraperitoneally. Very recently Mahmoud et al (1977), in his experiments, observed a generation of eosinophilopoietic factor (low mol. wt. peptide) during AES (Anti eosinophil serum) induced eosinopenia, which stimulates eosinophilopoiesis in-vivo. They named it as eosinophilopoietin.
In our preliminary experiments to establish the existence of eosinophilopoietin in tropical eosinophilia, we have studied a rat animal model. Eosinophilia was induced in rats by i.v. injection of HGG (human gamma globulin) coated latex particles. Eosinophils were observed to peak in peripheral blood on the 6th day and in the bone marrow on the 4th day suggesting new generation of cells in the bone marrow. This factor which is believed to induce eosinophilia could be transferred to normal rats by taking serum of the previous group and inducing fresh formation of eosinophils in -the latter group. Serum of tropical eosinophilia patients contains such a factor as evidenced by a rise of bone marrow eosinophils in rats injected with such sera, although eosinophilia is not seen in the peripheral blood. Further studies would prove whether the parasitic infection could increase the levels of eosinophilopoietin. The exact nature of stimulus to the formation of this factor, its chemical characteristics and the site of synthesis are still unknown.
Little is known of the movements of eosinophils, their sites of localization and their behavior under conditions which change the internal mileu either by drugs or by sensitization to antigens. Such kinetic studies in animals are difficult to carry out because the number of circulating eosinophils in normal animals is very low. However, it is possible to produce sustained eosinophilia in the animals and harvest a large number of cells for labeling purposes.
A subject with hypereosinophilia is an attractive model for studying the kinetics of eosinophils but such investigations have not been frequently carried out because a suitable gamma emitting radio-nuclide for labeling these cells has not been available.
To overcome these difficulties we carried out some experiments in the following aspects.
(i) Development of experimental eosinophilia and labeling of eosinophils with radionuclide
According to the previous reports by Litt, it is possible to produce eosinophilia in guinea pigs, with repeated intraperitoneal injections of horse serum. Labeling of these eosinophils (so that their movements can be easily traced) was attempted with several radionuclides such as 51Cr, 113mIn 153GD and 99mTc using various labelling technics., ,  Very recently a technic has been described by Thakur et al (1977), which yields high labelling efficiencies without disturbing the biological characteristics of the cells which is an essential prerequisite for any study. This procedure was followed for labelling eosinophils in our studies.
(ii) Kinetic studies
The distribution and kinetics of labelled eosinophils were followed in three groups of guinea pigs, (i) normal guinea pigs, (ii) guinea pigs who were administered diethylcarbamazine (DEC) or hydrocortisone, (iii) guinea pigs immunized against tetanus toxoid.
Such simulations of altered conditions existing in human diseases were attempted in the guinea pigs to understand the effect of these alterations on the distribution kinetics of the cells. The studies show that it is possible to label eosinophils with 113mIn-oxine without affecting their viability. Different experimental conditions produce changes in the distribution of eosinophils, the main reservoirs 'for which appear to be the lungs, liver, spleen, bone and muscle. In an animal actively immunized against antigen the distribution was predominantly in the RES system. Under the influence of drugs like DEC, again, there was a marked shift in the distribution patterns of eosinophils when compared to the controls.
Thus -tropical eosinophilia is a clinical syndrome, and though the filarial parasite is an important cause, other paraties whose life cycle takes them through lungs may also, on occasion, produce hypersensitivity reactions.
The very high IgE levels suggests an immediate type of hypersensitivity reaction in tropical eosinophilia. Bronchial spasm, mucosal oedema, increased secretion of mucus, mucosal sloughing which all contribute to airways obstruction and to asthmatic symptoms, may thus result from an IgE mediated response to the parasite. The increased blood levels of histamine and the high plasma histaminase activity in these patients further support the presence of a hypersensitivity reaction.3 Reagenic antibodies may play a secondary role by inducing immediate type hypersensitivity phenomenon which helps IgG class antibodies to expel the parasites. IgE antibodies are fixed to mast cells and basophils by special receptors and in the presence of antigen release vasoactive amines from mast cell granules like histamine, SRS-A, eosinophil chemotactic factor-A, serotonin which lead to manifestation like bronchial spasm. Also it is likely that eosinophilic chemotactic factor of anaphylaxis ECF-A, intermediate mol. wt. peptide chemotactic factors, and other anaphylactic mediators are involved in the local accumulation of eosinophils in parasite infested tissues., , ,  These eosinophils are responsible for the degradation of the chemical mediators released by mast cell degranulation.,  It has been shown that eosinophil histaminase and eosinophil arylsulphatase which is of B type inactivates histamine and SRS-A in a time and dose dependent reaction. The finding of an elevated levels of arylsulphatase B activity per circulating eosinophils at the height of filarial infection and the return of enzyme level to the normal range after successful treatment has suggested that the eosinophils mobilized with parasitic infestations may undergo an adaptive modification to improve its regulatory functions.
High circulating levels of eosinophils have been reported in many allergic conditions. It has been also reported that in contrast to eosinophils from normal subjects, eosinophils from patients with high number of circulating eosinophils are apparently inactive. The possible reason would be that eosinophils get altered possibly by interference of cell receptors by antigen antibody complexes at the local tissue sites. The used eosinophils thus then pour into circulation en route to the grave yard.
The presence of hypersensitive immune response of IgE and cell mediated sensitization to filarial antigen suggest that the syndrome of tropical eosinophilia is mostly due to a host reaction to the filarial antigen with an associated eosinophilic response. It may be an aberrant form of filariasis because similar reactions were observed in patients of acute filarial lymphangitis suggesting an identical host response but with no characteristic symptoms of tropical eosinophilia.
Understanding of the eosinophils and the eosinophilia has come a long way from the original pointer to therapeutic effectiveness of diethylcarbamazine in this syndrome reported first from the Seth G.S. Medical College. The Silver Jubilee of the Journal emanating from this Institution was a fitting occasion to retrace the course of these investigations covering the last 25 years.