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AMER. ZOOL., 23:185-194 (1983)
Phagocytosis and Encapsulation: Cellular
Immune Responses in Arthropoda1
STUART RATNER AND S. BRADLEIGH VINSON
Department of Entomology, Texas A&M University, College Station, Texas 77843
SYNOPSIS. The least understood aspects of the cellular immune reactions of arthropods
are the earliest events: the initial recognition of foreignness, and the resulting changes in
hemocyte behavior and morphology. There are indications that the recognition of a
surface as foreign is based primarily upon its electrostatic charge, but more specific criteria
may be utilized in some situations. Phagocytes are capable of recognizing foreignness
without the intervention of soluble opsonins but, in some arthropods, there is in vitro
evidence that opsonins can increase the efficiency of phagocytosis. It has been hypothesized, on the basis of ultrastructural evidence that encapsulation, a multicellular immune
response, is induced when labile hemocytes rupture upon encountering a foreign object.
The released products may promote the formation of a sheath of ameboid hemocytes
around the object. This hypothesis is now supported by the results of experiments performed on an in vitro encapsulation system. This system may prove useful in the purification
of encapsulation—promoting factors; in determining the mechanism of their release from
hemocytes; and in investigating the possibility that the various cellular immune reactions
of arthropods have a common underlying mechanism.
unicellular defense reaction which clears
the hemolymph of bacteria and other
The reactions of arthropodan hemo- microscopic foreign objects; and cellular
cytes against foreign bodies can be consid- encapsulation, one of the multicellular
ered to constitute an "immune system" only reactions, which is directed against larger
in the most general sense of the term: they invaders of the hemocoel (i.e., those greater
aid in keeping the animals free of parasites than 10 Mm in diameter [Salt, 1970]). The
and pathogens. The connotations which morphological aspects of these reactions
have become irreversibly bound to the term are becoming increasingly clear, but our
"immunity" as it has been used by verte- understanding of their physiology and biobrate-oriented immunologists—specific- chemistry lags far behind (see recent
ity, memory, the involvement of immu- reviews: Schapiro, 1975; Ratcliffe and
noglobulins—do not apply to the situation Rowley, 1979; Baerwald, 1979; Lackie,
in arthropods. No immunoglobulins are 1980). Some of the most difficult unrepresent, nor do the hemocytes possess solved questions deal with the earliest
receptors for them (Anderson, 1977). Dis- events in the cellular reactions: the actual
crimination is generally so poor that allo- recognition of foreignness and the prografts, and sometimes even xenografts, are cesses directly triggered by this recognitolerated (Lackie, 1981a). Although there tion. In this review we will survey the proare indications of specific memory in some gress which has been made toward
humoral reactions (Karp and Rheins, 1980), answering some of these questions. We will
no convincing evidence of cellular immune also consider the possibility that phagocymemory has been produced. Nevertheless, tosis, encapsulation, and other cellular
the cellular immune system evidently does immune responses share common underan adequate job of eliminating foreign lying mechanisms. Recent findings we have
material from the arthropodal hemocoel, made using an in vitro encapsulation system
judging from the success of the phylum.
will be incorporated wherever necessary.
Arthropodan hemolymph contains an
This review concerns phagocytosis, the
array of humoral immune factors, many of
which are antimicrobial (see Boman and
1
From the Symposium on Comparative Aspects of Hultmark, 1981). In this review, only
Inflammation presented at the Annual Meeting of the humoral factors associated with cellular
American Society of Zoologists, 27-30 December immunity will be considered.
1981, at Dallas, Texas.
INTRODUCTION
185
186
S. RATNER AND S. B. VINSON
classes, as suggested by Scott (1971); or it
Arthropodan phagocytes are morpho- may be merely a reflection of the use of
logically diverse, and have been granted different techniques, in a small number of
such names as granulocyte, amebocyte, experiments, upon a limited range of
lamellocyte and, perhaps most commonly, species.
Recently, there have been preliminary
plasmatocyte. For convenience, the term
"plasmatocyte" will be used in this paper indications of humoral mediators of
to denote all ameboid, phagocytic hemo- phagocytosis in insects which could operate by means other than opsonization.
cytes.
Mohrig and Schittek (1979) have isolated
Are humoral factors involved in phagocytosis'? from the hemolymph of Galleria mellonella
Opsonins are serum substances which, (L.) a complex of factors induced by the
when attached to foreign objects, increase injection of readily encapsulated particles
their susceptibility to phagocytosis. In ver- (latex beads). These factors confer upon a
tebrates, immunoglobulin G and the third naive larva the ability to clear its hemocomponent of complement act as opsonins. lymph of the normally unphagocytizable
Arthropod serum lacks immunoglobulins Bacillus thuringiensis subtoxicus. It remains
and complement (Whitcomb et al., 1974), to be proven, however, that this clearance
is accomplished by phagocytosis. Wago
but does it contain opsonic factors?
The standard test for serum opsonins is (1980) found that a component of Bombyx
the addition of serum-treated or untreated mori (L.) serum causes the elongation of
test particles (often mammalian erythro- hemocytic filipodia, an action which could
cytes) to hemocyte suspensions or mono- facilitate phagocytosis.
Regardless of the role of serum factors
layers. If an opsonin is present, serum-pretreatment will increase the binding or it seems certain, from the evidence preuptake of the particles by the hemocytes. sented above, that phagocytosis in arthroThis test has revealed the presence of poda can proceed in their absence. Recopsonins in some crustaceans (McKay et al., ognition of foreignness must be made by
1969; McKay and Jenkin, 1970; Paterson a component of the plasmatocyte memand Stewart, 1974; Schapiro et al., 1977). brane. The possible mechanisms of this
There is also evidence that a component recognition will be discussed in a later secof the serum of the crayfish Parachaeraps tion.
bicarinatus aids in the elimination of bacCELLULAR ENCAPSULATION
teria in vivo (Tyson and Jenkin, 1973)
Most arthropods have the ability to form
although it was not definitely determined
that phagocytosis was the means of elimi- a tight, multilayered hemocytic capsule
nation. In most cases, some degree of around macroscopic parasites and pathophagocytosis or binding can occur even in gens. This process, called encapsulation,
often kills the offending organism, or at
the absence of serum.
In contrast to the situation in Crustacea, least restricts its growth and movement.
all reported attempts to demonstrate Theories advanced to explain the mortalopsonins in insects have been unsuccessful ity of encapsulated organisms include
(Rabinovitch and DeStefano, 1970; Scott, asphyxiation, buildup of wastes, and toxic
1971; Anderson et al., 1972; Wago and action of quinones (Salt, 1963;Nappi, 1977;
Ichikawa, 1979; Rowley and Ratcliffe, Poinar et al., 1979). The quinones are pre1980). This is true even in insects whose cursors of melanin which often forms in
serum agglutinates the test particles the capsule.
The structure of hemocytic capsules is
(agglutinins are opsonic in some molluscs
[Hardy et al., 1977]). The discrepancy in fairly consistent in all arthropods in which
opsonin occurrence between crustaceans it has been described. A typical capsule
and insects may be the result of different consists of 5-30 layers of hemocytes packed
selective pressures exerted on the two tightly together with few or no intercellular spaces. Desmosomes and gap and sepPHAGOCYTOSIS
CELLULAR IMMUNE RESPONSES IN ARTHROPODA
tate junctions are sometimes observed
between the cells (Baerwald, 1979). The
innermost cell layer adheres tightly to the
foreign surface, and is often extremely flattened against it. Many of the overlying layers are similarly flattened, so that the inner
portions of some capsules resemble a myelin sheath. Capsule morphology varies from
species to species in relatively minor aspects:
extent of cell flattening, cell necrosis, presence or absence of melanin, and amount
of extracellular material (Crompton, 1967;
Grimstone et al., 1967; Norton and Vinson, 1977; Poinar and Hess, 1977; Schmit
and Ratcliffe, 1977).
Capsule development
The cells which comprise a capsule are
mainly ameboid phagocytic cells which
resemble (and may actually be) plasmatocytes. Ultrastructural studies of the early
phases of encapsulation, however, indicate
that a different class of hemocyte is responsible for the actual recognition of foreignness and initiation of encapsulation. These
labile, generally nonphagocytic hemocytes
have been termed hyaline cells, cystocytes,
spherule cells, or, as they will be referred
to in this paper, granulocytes. The use of
a single term is for convenience; these cells
may not all be homologous (Price and Ratcliffe, 1974).
Ultrastructural evidence indicates that
granulocytes rupture upon encountering a
foreign surface. The released granular
and/or electron-dense flocculent material
seems to cause plasmatocytes to adhere to
the foreign surface, and may contain the
chemical signal which induces them to flatten and conjoin to form the capsule. The
material may also contain enzymes and
precursors for melanin synthesis (Reik,
1968, as summarized by Salt, 1970; Nappi
and Streams, 1969; Unestam and Nylund,
1972; Brehelin et al., 1975; Schmit and
Ratcliffe, 1977, 1978). The material
released by the granulocytes will thus be
referred to as "encapsulation-promoting
factors" (EPF).
What is the nature of EPF?
Despite the important effects of EPF
upon cellular behavior, their composition
187
has been characterized in only the most
general terms. The labile hemocytes of
arthropoda have been demonstrated histochemically to contain acid or neutral
mucopolysaccharides and glyco- or mucoproteins (Ashhurst and Richards, 1964;
Dumont et al., 1966; Gupta and Sutherland, 1967; Wood?/al., 1971). Reik (1968;
summarized by Salt, 1970) implicated
mucopolysaccharides in the early stages of
capsule formation in Ephestia kuehniella
Zeller. A precise determination of the
nature of EPF and their action upon target
cells awaits purification of EPF, a feat
which would require an unambiguous
bioassay. We have recently developed a
technique which may fill this requirement,
and would like to digress briefly in order
to describe it.
An in vitro encapsulation system
Short-term culture of fresh hemolymph
of Heliothis virescens (F.) and H. zea (Boddie)
can be employed in the study of cellular
encapsulation in vitro. The hemolymph is
diluted 3-1 OX in Grace's insect tissue culture medium containing phenylthiourea at 8% of saturation. If foreign objects
(e.g., cotton thread) are present in the
medium, flocculent hemocytic aggregations form around them almost immediately. The aggregations become progressively denser and more sharply defined
until, at ca. 2 hr, a classic hemocytic capsule
surrounds the foreign object. At 5 hr, the
capsule bears strong ultrastructural resemblance to that formed in vivo by H. virescens
(Ratner and Vinson, in preparation).
When no foreign object is present, the
hemocytes aggregate spontaneously, eventually forming smooth, rounded nodules.
This response may be triggered by the foreign surface of the culture dish, or by factors exuded from injured tissues during the
bleeding process (see Cherbas, 1973). If
the addition of a foreign object is delayed
until after nodule formation is well under
way (ca. 30 min), the object is not encapsulated (Fig. 1 A). Thus, the system rapidly
becomes exhausted, probably because of
the commitment of cells, and perhaps other
components, to the nodules.
Both encapsulation and nodule forma-
188
S. RATNER AND S. B. VINSON
ADD
FOREIGN
OBJECT
FRESHLY DRAWN
HEMOLYMPH
+ MEDIUM
WITHHOLD
FOREIGN
OBJECT
AGGREGATION
OF HEMOCYTES
BEGINS
ENCAPSULATION
OF OBJECT
BEGINS
ADD FOREIGN
, .OBJECT
OBJECT NOT
ENCAPSULATED
sulation. Neuraminidase and hyaluronidase do not prevent encapsulation so, if the
in vitro system can be taken as a guide, the
crucial components of Heliothis EPF are
proteins, not mucopolysaccharides.
It should be possible to assay hemolymph
fractions for EPF activity, and eventually
purify an EPF, using trypsin-inactivated in
vitro systems.
What is the mechanism of EPF release?
It is desirable to know what cellular processes cause granulocytes to rupture after
the initial recognition of foreignness. This
knowledge might be used in devising a
method of inducing inappropriate EPF
release in pest arthropods, in effect turning
their own immune systems against them.
Some preliminary findings about the process have emerged from the in vitro Heliothis
system.
When the hemolymph culture is held at
4°C, the usual aggregation of hemocytes
still forms and clings to a foreign object,
but it will not mature into a tight, smoothFIG. 1. In vitro demonstration of an encapsulation- contoured capsule unless the culture is subpromoting factor released from hemocytes of Heliothis sequently warmed to room temperature.
virescens. A. Behavior of an untreated culture. B.
Inhibitors of glycolysis and oxidative phosBehavior of a gently trypsinized culture (Ratner and
phorylation also prevent capsule maturaVinson, in preparation).
tion, but not the initial aggregation of
hemocytes. It seems, then, that the rection can be prevented, however, by a brief, ognition of foreignness and consequent
mild trypsinization of the culture (soybean release of EPF are passive processes, while
trypsin inhibitor is used to control the the subsequent consolidation of the capduration of trypsinization) (Fig. IB). The sule requires metabolic energy.
ability to encapsulate can subsequently be
Capsule initiation is also unaffected by
restored to the culture by the addition of the presence of membrane-stabilizing drugs
lysate prepared by the ultrasonic disrup- such as corticosteroids and local anestion of fresh, whole hemolymph (Fig. IB). thetics (Ratner and Vinson, in preparaThe addition of freshly prepared serum or tion). Direct microscopic observation conGrace's medium has no restorative effect. firms that these compounds are ineffective
It therefore appears that mild trypsiniza- in preserving granulocyte structure in vitro.
tion prevents encapsulation not by altering Since the drugs are thought to act by discomponents of the hemocyte membrane, placing membrane-bound calcium and
but by digesting a protein released by the ATP-ase (Seeman, 1970), these substances
hemocytes—an EPF (stronger trypsiniza- are apparently not involved in granulocyte
tion does appear to damage the hemocyte rupture in Heliothis. Obviously, our knowlmembrane, since it renders the culture edge of the mechanisms of EPF release are
unable to encapsulate, even after the addi- at present rudimentary. Much may be
tion of lysate). This is, to our knowledge, revealed by tests of other membrane-active
the first experimental evidence that such compounds. Such tests are best done in vitro,
a factor can actually play a role in encap- to ensure that the compounds reach the
FRESHLY DRAWN
HEMOLYMPH
+ MEDIUM
+0.05% TRYPS1N
+ FOREIGN OBJECT
ADO
LYSATE
ADO
SERUM
OR
MEDIUM
OBJECT NOT
ENCAPSULATED
INCUBATE
15 M1N
I ADDTRVPSIN
I INHIBITOR
ENCAPSULATION
OF OBJECT
BEGINS
I INCUBATE
I 2 KR
OBJECT NOT
ENCAPSULATED
]
'
INCUBATE
12 HR
OBJECT
NOT ENCAPSULATED
CELLULAR IMMUNE RESPONSES IN ARTHROPODA
189
Nolan, 1980). This finding is consistent
with Salt's hypothesis since, at physiological pH, basement membrane is negatively
charged (Vinson, 1974). Also negatively
WHAT ARE THE PRINCIPLES OF NON-SELF
charged are some of the coatings and memRECOGNITION IN ARTHROPODA?
branes which protect parasites from encapWe have seen evidence that two types of sulation (e.g., Brennan and Cheng, 1979).
arthropodan hemocytes react to a foreign
It is not known whether surface charge
surface: the phagocytic plasmatocytes and similarly affects phagocytosis by arthrothe labile granulocytes. What aspects of a podan hemocytes. It is interesting that catsurface do these cells actually recognize as ionic substances increase the phagocytosis
foreign? Salt (1970) hypothesized that, to of bacteria by human neutrophils in an
an insect immune system, "non-self is opsonin-free system (Pruzanski and Sato,
equivalent to anything other than intact, 1978). In a different system, bacteria with
native basement membrane. His hypothe- surfaces more hydrophobic than neutrosis stemmed from the fact that, except for phils were phagocytized more readily than
parasites and pathogens which have devel- those with more hydrophilic surfaces (Van
oped specific counteracting strategies, few Oss and Gilman, 1972). It should be posforeign surfaces prove immunologically sible to use similar experiments to deterinert to arthropods. Even materials such as mine the effect of surface charge on phagoglass and polyfluorocarbons elicit a cytosis by arthropodan hemocytes.
response. Normally tolerated allografts,
A model of non-self discrimination based
however, are encapsulated if their baseon
nonspecific physicochemical factors can
ment membrane is damaged. Salt further
adequately
account for all known properhypothesized that the immunologically
important components of basement mem- ties of arthropodan cellular immune sysbrane are acid mucopolysaccharides. There tems. For example, in a single host, the
have been some criticisms (Crossley, 1975), severity of encapsulation of transplanted
but Salt's hypothesis is still the best avail- tissue can vary according to the species of
able. It does not, however, address the spe- the donor (Lackie, 1981a; S. E.Jones, percific molecular principles of non-self rec- sonal communication). These differences
in severity could be related to the number
ognition.
of granulocyte ruptures provoked by each
transplant. This in turn could be a reflecInitial recognition offoreignness
tion of different distributions and strengths
Since the arthropodan immune system of charged groups on the basement memcan react against inorganic surfaces, it is branes of the various donors (Lackie,
obvious that the initial recognition of for- 1981a). Recognition in a nonspecific syseignness cannot be explained completely tem need not occur only at the hemocyte
by models involving lock-and-key interac- membrane. Soluble factors could stick to
tions between complex biogenic molecules surfaces with foreign physicochemical
(e.g., the glycosyl transferases of Roseman, characteristics, and bind to hemocytes by
1974, and Parish, 1977). Such highly spe- cytophilic groups.
cific factors may come into play in secondThe fact remains, though, that arthroary reactions, or in special cases, as we shall podan hemolymph does contain molecules
see below; but the initial recognition of which have highly specific binding affiniforeignness must involve less specific, ties. Hemagglutinins, with their saccha"physicochemical" properties of a surface. ride-specific binding, are an example (YeaElectrostatic charge seems to be such a ton, 1981). What role could such molecules
property. A growing body of evidence sug- play in an immune system which shows so
gests that hemocytes form capsules on pos- little in vivo specificity? One possiblity is
itively charged surfaces, but not neutral or that they are secondary factors, released
negatively charged ones (Walters and Wil- or activated only after the initial recogniliams, 1966; Vinson, 1974; Dunphy and tion of foreignness. For example, one would
hemocytes in known forms and concentrations.
190
S. RATNER AND S. B. VINSON
expect that EPF must bind specifically to
some components of the plasmatocyte
membrane, since capsules are composed
almost exclusively of these cells. This specific binding could not occur, however,
until the EPF is released by granulocyte
rupture, i.e., after foreignness has been
recognized. Since granulocytes are
extremely fragile, it would not be surprising if their released factors showed up as
serum components in extracted hemolymph.
Another possibility is raised by Lackie
(1981a), who suggests that specifically
binding factors actually can participate in
the initial recognition of foreignness. In
Lackie's "two-tier" hypothesis, the nonspecific recognition system—the first
"tier"—could be supplemented by factors
which trigger cellular responses against
more specific stimuli. A two-tier system is
an entirely plausible concept. It might arise
if the physicochemical surface properties
of a damaging parasite were close enough
to those of the host to prevent recognition
by host granulocytes. One possible consequence of the resulting selective pressure
upon the host might be the development
of a receptor which could recognize other,
possibly more specific, aspects of the parasite's surface. This new receptor could be
soluble or bound to hemocyte membranes.
What components of arthropodan hemolymph might act as specific recognition
factors in the type of system proposed by
Lackie? One possibility is a group of
enzymes, the phenoloxidases, which catalyze the oxidation of tyrosine and other
phenolic compounds into dopa, a precursor of melanin. A connection between
phenoloxidases and encapsulation has long
been suspected, since inhibitors of melanin
formation have sometimes proven inhibitory to encapsulation as well (Salt, 1956;
Brewer and Vinson, 1971; Nappi, 1973;
Beresky and Hall, 1977). The hemolymph
of some arthropods contains an inactive
prophenoloxidase which can be activated
by fairly specific stimuli such as fungal wall
polysaccharides and yeast zymosan (Pye,
1974; Soderhall and Unestam, 1979: Ashida and Dohke, 1980). Interestingly, the
activated enzyme adheres readily to fungal
cell walls and abiotic surfaces such as glass.
The inactive proenzyme is not adhesive
(Soderhall et al, 1979). Phenoloxidases,
then, exhibit the type of behavior one
would expect from an arthorpodan nonself recognition factor, although it remains
to be proven whether they actually perform this function. The fact that the phenoloxidases and their substrates are often
found localized within hemocytes may
mean that they come into play only after
granulocyte rupture.
Another class of molecules which might
act as the "specific tier" in a two-tier system
is the hemolymph agglutinins. Many agglutinins bind polyvalently to specific oligosaccharide residues, which is why they are
often detected by their ability to clump
mammalian erythrocytes and bacteria. In
some molluscs, agglutinins have been
proven to act as opsonins (Hardy et al.,
1977). In arthropoda, only circumstantial
evidence links agglutinins with cellular
immune response. The most convincing
evidence was obtained by Lackie (19816,
c). Comparing an insect with low xenograft
tolerance (Periplaneta americana [L.]) to one
with greater tolerance (Schistocerca gregaria
Forsk.), Lackie found that the insect with
better non-self discrimination possesses a
wider spectrum of specific agglutinins. Even
more intriguing was the finding that the
ability to agglutinate Hymenolepis diminuta
oncospheres in vitro is correlated with the
ability to encapsulate them in vivo. Lackie
suggests that the agglutinins might act as
hemocyte-bound receptors, an idea partly
inspired by the immunofluorescent localization of Leucophaea maderae Fabr. agglutinin on hemocyte membranes (Amirante
and Mazzalai, 1978). Definitive proof of
the involvement of arthropodan agglutinins in the recognition of foreignness is still
lacking. The blocking of phagocytosis by
antibody to purified agglutinins would constitute such a proof.
A third possibility must be considered:
The specifically binding hemolymph factors may not be involved in cellular immunity at all. Instead, they may constitute a
strictly humoral antimicrobial system, or
they may perform presently unsuspected
functions unrelated to immunity.
CELLULAR IMMUNE RESPONSES IN ARTHROPODA
191
question, for if distinctions between the
reactions are only superficial, then coordination between experimenters is needArthropodan hemocytes participate in a lessly inhibited. Experimental designs may
variety of multicellular reactions other than even be distorted. For example, in the in
encapsulation. In nodule formation, con- vitro Heliothis system, there seems to be
glomerates of bacteria or other particu- equivalence between the immune and
lates are sealed into a hemocytic sheath hemostatic reactions. If no foreign object
(Ratcliffe and Gagen, 1977). In certain is present, a nebulous clot forms and eventypes of coagulation, hemocytes and extra- tually contracts into a nodular mass. If a
cellular materials form a mass which can foreign object is present, the clot forms
be an early stage in the process of wound around it and matures into a capsule. Thus,
repair (Clark and Harvey, 1965; Gregoire, if the "clot"were allowed to form before
1974). The formation of so-called "tumors" the start of an experiment (a practice not
in insects is, in most cases, the walling off unknown in in vitro studies of hemocyte
of pathological tissues by hemocytes (Pe- behavior), the remaining culture would be
seriously depleted of immunoreactive cells.
rotti and Bairati, 1968).
It has often been suggested that these
One way to test for similarities among
reactions are interrelated, since they all the mechanisms of the cellular responses
show similar patterns of development is to look for similarities among the extra(reviewed in insects by Ratcliffe and Row- cellular factors associated with those
ley, 1979; for crustaceans, see Schapiro, responses. Two types of factors which have
1975, and Ghidalia et al., 1981). The first often been analyzed are the clottable proevent is usually the release of the contents teins and the hemagglutinins, so they might
of labile hemocytes at or near the site of be good subjects for comparison. At presthe triggering stimulus. Mucopolysaccha- ent, though, it is hard to draw comparirides and mucoproteins have been detected sons, because of the diverse analytical
histochemically in the released material. methods used by different groups of experNonlabile hemocytes are subsequently imenters. The only generalization that
ensnared in, or perhaps attracted to, this emerges from a survey of the literature is
material. These hemocytes eventually that both types of factor tend to be proteins
become organized into a multi-layered of molecular weight 200,000-500,000,
sheath. There are exceptions (Crossley, composed of noncovalently linked subunits
1975), but that is the general pattern, based with weights of 20,000-50,000 (Marchalprimarily on electron microscopic obser- onis and Edelman, 1968; Doolittle and
vations. There are also experimental results Fuller, 1972; Hall and Rowlands, 1974;
which suggest that a variety of cellular Donlon and Wemyss, 1976; Shishikura et
immune reactions are all potentiated by al., 1977; Brehelin, 1979). Obviously,
substances liberated from hemocytes. meaningful comparisons are impossible at
Hemocyte lysate promotes encapsulation present. They await the purification of
in the in vitro Heliothis system (Ratner and more factors, including EPF, as well as a
Vinson, in preparation); simulates in vitro coordination of analytical techniques.
clotting of the hemolymph of the crab CarAnother way to approach the question
anus maena (Bang, 1971); and, when coated of interrelationships between cellular
on bacteria, speeds their clearance (pre- immune response mechanisms is to comsumably phagocytic) from the circulation pare the metabolic activities associated with
of the crayfish P. bicarinatus (Tyson and them, especially the activation of biochemJenkin, 1973).
ical pathways. This strategy was employed
None of this information provides an by Anderson et al. (1973) in their comparanswer to a fundamental question: To what ison of phagocytotic processes in inverteextent do all the cellular reactions, includ- brates and mammals. It has not yet been
ing phagocytosis, share a common under- applied to a comparison of arthropodan
lying mechanism? This is an important multicellular reactions.
INTERRELATIONSHIPS BETWEEN
CELLULAR IMMUNE RESPONSES
192
S. RATNER AND S. B. VINSON
PROSPECTS FOR FUTURE RESEARCH
It must by now be obvious to the reader
that our present understanding of the early
events in arthropodan immune response is
rudimentary. We have a fairly good ultrastructural picture of what is going on, but
tentativeness creeps into even the most
general statements about the molecular
nature of non-self recognition, and consequent hemocytic changes.
Tools now becoming available can and
should be used to deepen our knowledge
of the mechanisms of arthropodan immune
response. A variety of charged groups can
now easily be attached to inert substrates
(see, for example, Carlsson et al., 1979).
This should make possible a finer determination of the effect of surface charge on
phagocytosis and encapsulation. Investigations of reactions against transplants are
growing more numerous, and more subtle.
It would be helpful if biochemical analyses
of the transplant surfaces could be coordinated with these investigations. Another
research opportunity is afforded by the
viruses associated with some hymenopterous parasitoids of insects. These viruses
suppress the host's immune response to the
parasitoid's egg (Edson et al., 1981). The
determination of their mode of action may
reveal underlying mechanisms of that
response.
In vitro systems have proven valuable in
the detection and study of the serum and
cellular factors involved in phagocytosis. A
high priority should now be placed upon
the improvement of methods of delaying
and controlling the multicellular immune
reactions in vitro, so that their associated
factors, such as EPF, can be isolated, purified, and tested. Such in vitro systems will
also make it possible to determine whether
substances which inhibit multicellular
reactions in vivo (e.g., phenoloxidase inhibitors and viruses) act directly upon the
hemocytes, or by some indirect mechanism.
ACKNOWLEDGMENTS
Approved as TA 16787 by the Director,
Texas Agricultural Experiment Station. All
programs and information of the Texas
Agricultural Experiment Station are avail-
able without regard to race, ethnic origin,
sex, or age. Supported in part by NSF Grant
PCM-8021646.
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